586 



TELEOSTEI 



CHAP. 



Gastromyzon of North Borneo, in which the pectoral and ventral 

 fins are much expanded to form, with the belly, a sucker by 

 which the fish adhere to the stones of mountain torrents, showing 

 a remarkable analogy to Exostoma among the Silurids. 1 



FIG. 355. Gastromyzon borneensis, ventral view, natural size. 



Fam. 4. Siluridae. Mouth non-protractile, bordered by the 

 praemaxillaries and the maxillaries, or by the praemaxillaries 

 only, the maxillaries being often rudimentary and supporting the 

 base of a barbel ; jaws usually toothed. Parietal bones usually 

 confluent with the supraoccipital, forming a single large plate 

 (parieto-occipital) ; symplectic and suboperculum absent. Pha- 

 ryngeal bones normal, with small teeth. Eibs attached to the 

 lower surface of long parapophyses ; epipleurals absent. Pectoral 

 fins inserted very low down, folding like the ventrals, often armed, 

 like the dorsal, with a strong bony spine. Body naked or with 

 bony plates. An adipose dorsal fin often present. One to four 

 pairs of barbels. 



The skull and the opercular apparatus show a reduction in the 

 number of elements as compared with the Characinids and 

 Cyprinids, such as the absence of the metapterygoid, the often 

 rudimentary, rod-like condition of the palatine, and the fusion of 

 the parietals with the supraoccipital. 2 The scapular arch is 

 solidly united to the skull and is often very massive, and the 

 occiput may be connected with the base of the dorsal fin by a 

 buckler formed by the expansion of the first and second inter- 

 neural bones. The pterygials or supports of the pectoral rays 

 are large and reduced to two or three. 3 Teeth are rarely present 



1 On the anatomy of the Cyprinids, of. Sagemehl, Morphol. Jahrb. xvii. 1891, 

 p. 489. 



2 Cf. Boulenger, " Poissons du Bassin du Congo," p. 238 (1901). 



3 In Exostoma these bones are two in number and so elongate as to resemble the 

 condition characteristic of the Pediculati. 



