114 



Records of the Indian Museum. 



[Vol. XVI, 



The loach resembles other small species of its genus in general 

 structure but has the processes between the two nostrils on each 

 side of the head prolonged into regular barbels. This feature 

 does not occur in any other Indian species of N emachilus but is 

 present in the closely allied genus Aborichthys, which is found in 

 hill streams in /(ssam and north-eastern Burma. 



The modifications exhibited by the Discognathus and the 

 Psilorhynchus ^re of a more extreme and more clearly adaptive 

 nature. Many species of N emachilus, with their ventral mouth, 

 swollen suctorial lips, glandular skin, flat ventral surface and small 

 size are well fitted to live in rapid-running water among rocks, to 

 which they are able to adhere and over which they can crawl 

 even when the surface is not entirely submerged ; but other species 

 of similar structure live in ordinary rivers. The two species of 

 Psilorhynchus are inhabitants only of streams in or near Indian 

 mountains, and are modified accordingly. The Bombay species does 

 not differ from its Himalayan congeners to any important degree 

 in this respect. It has been generally assumed that the genus 



Fig. I. — Vertical section through the mouth of DiscogiiatJitts lamta. 



b.c. =■ buccal cavity. /./. =: lower jaw. /./. =^ anterior free portion of lower lip. 

 m.d. ^=. mental disk. u.j. ^ upper jaw. ii.l. = upper lip. 



Discognathus has had a similar history, but though Psilorhynchus 

 may be a more primitive form of Discognathus, there is something to 

 be said for another view. 



The main if not the only generic peculiarities of Discognathus 

 lie in the structure of its mouth and lips (fig. i). The jaws are of 

 semicircular shape, firm and bony, with blunt edges and covered 

 with soft integument; they are directed downwards and can be 

 tightly closed together with their margins in contact. The upper 

 lip is a deep but delicate fold covered with minute papillae, mobile 

 and probably sensitive. It entirely conceals the upper jaw, to 

 which it is not attached. The lower lip bears much the same 

 relation to the lower jaw so far as its anterior part is concerned, 

 but the free portion is shorter ; posteriorly it is much enlarged 

 and flattened and bears in its centre a relatively large transversely 

 oval or nearly circular pad, which can be elevated by muscular 

 action and so produce a partial vacuum, thus forming an organ of 

 adhesion. The mouth of Psilorhynchus is essentially similar, 

 except that the pad is entirely absent or represented, as in P. ten- 



