igig.] N. Annandale: Bombay Streams Faima. 117 



resembles D. nasutus in this respect, so far as can be judged from 

 published figures. 



Taking all these facts into consideration and further remem- 

 bering that the peculiar structure of the pectoral fins found in 

 Psilorhynchus is also found in Homaloptera and to a still greater 

 degree of specialization in the Bornean genus Gastromyzofi,^ I am 

 of the opinion that the resemblance between Psilorhynchus and 

 Discognathus is to a large extent due to parallel evolution. In 

 the species of the latter genus that live in comparatively still 

 water {i.e. in the majority of species) it is superficial, while 

 in those species that live in hill torrents convergence has 

 certainly taken place. I am inclined to think, therefore, that 

 both Psilorhynchus and Discognathus have been derived from 

 a genus like Crossochilus or probably Labeo, but that their evolu- 

 tion took place independently, aiid that whereas the chief factor 

 in the case of Psilorhynchus was rapid-running water in a rocky 

 stream-bed, in Discognathus the primary factor was a peculiar 

 mode of feeding. The close resemblance between such forms as 

 P. tentaculattis and D. nasutus is due, if this be so, to secondarv 

 convergence. The resemblance between D. quadrimaculatus and 

 Crossochilus may or may not be due to a real genetic relationship, 

 while that between the former and Psilorhynchus , close though it 

 seems at first sight, cannot be closer at most than that between 

 distant cousins, so far as descent is concerned. 



Molluscs. — The only mollusc found in the upper parts of the 

 streams at Khandalla was Ampullaria nux. As we have pointed 

 out in our systematic notes on the MoUusca collected on my tour, 

 this species is modified for its peculiar habitat in respect to two 

 characters: (i) its small size and {2) the curious development of 

 the inner lip of the aperture of the shell. It is the only species of 

 Ampullaria that I have seen in anything but still or at most 

 very sluggish water. The columellar callus of the shell is a flat- 

 tened ridge nearly two millemetres broad, and this ridge forms with 

 the outer lip a continuous margin of attachment, enabling the 

 shell to come into much closer contact with the rocks to which the 

 animal attaches itself than is the case with the shells of more 

 normal species that crawl on water-weeds. Only one living indivi- 

 dual was seen, though broken shells were not uncommon in the 

 streamlets and it is probable that A. nux, like many other species 

 of its genus, conceals itself in the dry season. The one individual 

 was attached to the rocky margin of a small pool densly shaded at 

 all times of day and supplied by a small but perennial waterfall. 



Paludomus obesa was found in some abundance in the same 

 streams, but in a small plain where it was not shaded and had 

 lost for a short distance its peculiar character as a mountain 

 streamlet. It was feeding on algae covering stones. 



Insects. — As I have already stated I can deal only with the 

 Rhynchota of the streamlets. These fall from a bionomical point 



' See Weber, Iiido-Aiistralian Fishes, III, p. 3, fig i (1916). 



