Zoologische Mededelingen, 83 (January 2009)P. Bleeker: A precursor of the fishes of the Indian Archipelago. Part 1 – Siluri
Subfamily V SILURICHTHYOIDEI

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Hemisilurus Blkr. highback

No dorsal fin. Anal and caudal fins not confluent. Two maxillary barbels. Teeth acute, short, the upper jaw teeth multiple-rowed, the vomerine teeth placed in 2 oval, separate groups. Branchiostegal membrane has 9 to 13 rays. Interbranchial membrane with a deep incision. Eyes covered with head skin, placed almost behind the mouth corner. Anterior nostrils tubular. Bone terminal.

Remark. In my earlier description of the species, from which the genus Hemisilurus was taken, I erroneously described 4 barbels. The less good state of preservation of the single specimen then available to me must have misled me in the recognition of the lower jaw barbels. Probably I have taken torn pieces of skin or threads as barbels, however in a very well preserved specimen, which later came in my possession, I found no trace of lower jaw barbels.

The genus Hemisilurus already by its Habitus distinguishes itself from all other genera of Schilbeïnes. One can already recognise it by its high back. More essential generic characters however can be found in the dentition, the absence of the dorsal fin and the presence of only two barbels.

Because of the complete absence of a dorsal fin, Hemisilurus is related to the genera Micronema and Phalacronotus, however, it has a totally different Habitus, whereas in the genera Micronema and Phalacronotus the vomerine teeth are placed in an undivided transverse band.

296 Because of the arrangement of the vomerine teeth into two separate groups, Hemisilurus reminds one of the genera Wallago, Belodonthichthys and Pseudosilurus, but apart from the fact that the peculiarities of that arrangement and the size and shape of that of the mentioned genera are different, the absence of the dorsal fin as well as the presence of only two barbels set it apart from them.

The only species of Hemilurus known to me till now are Hemisilurus heterorhynchus and Hemisilurus schilbeides. They differ remarkably from each other and can easily be distinguished by the following characters.

I.

Snout nearly truncate. Eyes contained 4⅓ to 5 times in length of head. Barbels reaching the eye. A. 90 to 93.

Hemisilurus heterorhynchos Blkr.

II.

Snout very depressed. Eyes contained 2½ times in length of head. Barbels reaching the anal fin. A. 65 to 67.

Hemisilurus schilbeides Blkr.


Hemisilurus heterorhynchos Blkr., Xenognathic highback

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Atl. Silur. tab. XLV fig. 2.

FIG2

Fig. 75. Hemisilurus heterorhynchus Blkr.

A Hemisilurus with an elongate, compressed body, body width contained about 2½ times in its depth, body depth contained 4⅕ to nearly 5 times in its length without, and 5⅕ to 5¼ times in its length with caudal fin. Head slightly depressed, compressed, contained 6 to 5⅘ times in length of body without, and nearly 7 to 6⅗ times in length of body with caudal fin; depth of head contained 1¾ to 1½ times in its length, width about twice; eyes nearly posterior, eye diameter contained 4½ to 5 times in length of head, distance between the eyes a little more than once to 1½ times their diameter; rostro-dorsal profile concave at the forehead and crown, very convex at the nape. Head shield entirely divided by a longitudinal groove entering the very thin supraoccipital crest; snout nearly truncate, rostral profile very obtusely rounded; anterior nostrils close to the tip of the snout, with well visible small tubes; jaws nearly equal, the upper jaw hardly longer than the lower jaw; mouth terminal. Jaw teeth multiple-rowed, acute, small, nearly equal, dense, upper jaw teeth placed in a curved, nearly rectangular band, lower jaw teeth in a band in the form of a horseshoe; vomerine teeth small, acute, placed in 2 separate, rounded groups, smaller than the pupil; barbels inserted in the vicinity of the nasal tubes, thin, reaching the eye; lips broad, membranaceous; chin with 2 conspicuous, separated pores towards the middle; gill 297 cover divided in two parts by a median, obtuse ridge inclining towards the base of the pectoral fin; humeral bone short, rounded at the top; lateral line straight, simple, characterized by touching small tubes; axillary slime pore not conspicuous. Instead of a dorsal fin there is a small, subcutaneous tubercle; pectoral fins slightly acute or slightly obtuse, not or hardly shorter than the head, spine thin, compressed, the bony part feebly denticulate; ventral fins acute or slightly acutely rounded, more than twice as short as the pectoral fins; anal fin more than 4 times as long as the head, ending before the base of the caudal fin; caudal fin incised nearly upwards from the base, with acute lobes, nearly equal, contained 7⅔ to a little more than 8 times in the length of the entire body. Colour of body dorsally olive, flanks and upper part slightly olive-coloured, lower part pearly. Fins yellowish-hyaline, more or less with dark speckles. Fleshy part of the anal fin silvery or pearly.
B. 12 or 13. D. 0. P. 1/17 or 1/16. V. 1/8. A. 90 to 93. C. 1/15/1 and shorter ones alongside.
Syn. Wallago heterorhynchos Blkr, Nieuwe Tient. Vischs. Sumatra, Natuurk. Tijdschr. Ned. Ind. V p. 514.
Hab. Sumatra orientalis (Palembang, Moarakompeh), in rivers.
Length of the 3 specimens 244”’ to 326’”.

Remark. I possess from the species in question three specimens, one caught in Moarakompeh, the two others in Palembang. In both places the species seems to be rare.

Hemisilurus schilbeides Blkr., Schilbe-like highback

Atl. Silur. tab. XLII fig. 4.

Fig. 76. Hemisilurus schilbeides Blkr. [In Atlas 98 mm TL.]

A Phalacron. [Hemisilurus] with an elongate, compressed body, body width contained about 2¼ times in its depth, body depth contained 3¾ to 4¼ times in its length without, and 4½ to a little more than 5 times in its length with caudal fin. Head very depressed, not compressed, acute, contained 6½ to 7 times in length of body without, and 7¾ to 8⅓ times in length of body with caudal fin; depth and width of head contained nearly 1½ times in its length, depth of the head above the angle of the mouth contained nearly 3 times in its length; eyes posterior, eye diameter contained about 2½ times in length of head, eyes more distant at the upper than at lower margin, distance between the eyes a little more than once their diameter; rostro-dorsal profile very concave at the head and anterior part of the nape. Head shield entirely divided by a longitudinal groove reaching the triangular, short, acute supraoccipital crest that is traversed by a groove; snout very depressed, rostral profile obtusely rounded; nostrils separate, the anterior nostrils close to the anterior edge of the snout, with short tubes; jaws equal; teeth acute, multiple-rowed, small, nearly equal, upper jaw teeth placed in a crescent-shaped band, lower jaw teeth in a band resembling the form of a horseshoe, vomero-palatine [vomerine] teeth in 2 very small groups, separated only a little; barbels thin, reaching the anterior rays of the anal fin; mouth about twice as broad as long, length contained about 4 times in length of head; lips broad towards the angle of the mouth; chin without conspicuous pores; gill cover without conspicuous ridge: humeral bone short, obtuse, rounded; lateral line straight, characterized by touching small tubes; axillary slime pore not conspicuous. No conspicuous tubercle instead of a dorsal fin; pectoral fins slightly obtuse, convex, longer than the head, spine thin, much longer 298 than the postocular part, posterior side with small teeth that are only visible with the aid of a lens; ventral fins rounded, more than 4 times or about 4 times as short as the pectoral fins; anal fin more than 5 times as long as the head, ending before the base of the caudal fin; caudal fin with a deep incision and acute lobes, the lower one hardly longer than the upper one, contained about 6 times in the length of the entire body. Colour of upper part of the body softly olive-hyaline, lower part pearly. Fins yellowish-hyaline, more or less with dark speckles.
B. 9 or 10. D. 0. P. 1/12. V. 1/5? A. 65 to 67. C. 1/15/1 and shorter ones alongside.
Hab. Sumatra (Palembang), in rivers.
Hab. Borneo (Bandjermasin), in rivers.
Length of the 2 specimens 96’’’ and 99’’’.

Remark. This Hemisilurus in various aspects diverges greatly from Hemisilurus heterorhynchus. There is not even a trace of a dorsal fin here, even the dorsal fin knob, which in the other species as well as in the species of Micronema and Phalacronotus indicates an origin of the dorsal fin, is lacking. Moreover, the barbels are extremely long, whereas they are shorter than the head in Hemisilurus heterorhynchus. For the rest the species in question is just as remarkable because of its very concave profile that reminds one of the genus Schilbe, whereas the rays of the branchiostegal membrane and of the pectoral and the anal fins are less numerous. I was not able to determine the number of pelvic fin rays in my specimen, however, I certainly have not seen more than 6 (1/5). The name of the species I have derived from the similarity in Habitus with Schilbe.

I received this species already a long time ago with a number of specimens of Kryptopterichthys laïs, from Banjermassin, and very recently also from Palembang. It seems not nearly to reach the size of Hemisilurus heterorhynchus.