554 Mr. C. T. Regan on the 



simulating a toothed maxillary. In DipJomystes the maxil- 

 lary is expanded distally and toothed, but in all other Silu- 

 roids it is toothless, serving only as the base of a barbel ; in 

 Eutrojjiichthys the small toothless maxillary bears a barbel 

 and articulates with the anterior end of the palatine just as 

 in the closely allied Schilbichthys. The palatine articulates 

 ■with the lateral ethmoid and bears the maxillary ; the ptery- 

 goid, when present, is small, connecting the palatine with 

 the mesopterygoid ; the metapterygoid is always well deve- 

 loped, suturally united with the quadrate and usually with 

 the hy ©mandibular. The operculum and interoperculum are 

 constantly present ; the lower pharyngeals are toothed 

 (except in some Loricariida), separate (except in Hypo- 

 phthalmus), opposed to a single pair of dentigerous patches 

 supported by the third and fourth pharyngobranchials, the 

 first and second being absent. 



The pectoral arch of the Siluroids is highly character- 

 istic ; the post-temporal, when present, is a small plate 

 rigidly attached to the skull, overlying the suture between 

 epiotic and pterotic and reaching the supraoccipital ; distally 

 it overlaps the proximal extremity of the supra-cleithrura, 

 which is typically forked, the upper limb usually rigidly 

 attached to pterotic and epiotic, the lower to the basi- 

 occipital ; sometimes the lower limb is absent (Clariidae, 

 Caliichthyidse, Loricariidte); the distal part, " stem,^^ of the 

 supra-cleithrum, beyond the fork, is deeply cleft to form 

 a socket for the head of the cleithrum. The mesocoracoid 

 is usually present, but is wanting in three families, Ariidie, 

 Doradidse, and Bunocephalidte ; the hypocoracoids usually 

 form an interlocking symphysis behind that of the cleithra, 

 but in certain groups (Siluridaj, Trichomycteridte) they taper 

 forwards below and do not form a symphysis. The pectoral 

 radials are three in number, the first short, the outer ones 

 more or less elongate. 



The centrum of the first vertebra is a disc, rigidly united 

 and often ankylosed to the basioccipital and to the complex 

 centrum, which is formed by the fusion of the second, third, 

 and fourth; the first parapophysis is that of the fourth 

 vertebra, corresponding to the os siispensorium of the 

 Cypriuoids; the fifth vertebra is rigidly attached to the 

 complex and its parapophysis also supports the air-bladder. 

 The sixth and following vertebrte usually bear ribs attaclud 

 to normal parapophyses, but the anterior [Pseudechoieis, 

 CalUclUkys, Dt)umeinai) ov oWi^Corydoras , Loricariidie, Buno- 

 cephalidui) the ribs may be sessile. 



The " nuchal shield," which is so characteristic of many 



