XVI INTRODUCTION. 



Planorbis, denticles and lamellae are also found on the inner walls 

 of the shell, these also being absorbed at a later stage ; thus 

 should an immature shell be split open these are often disclosed, 

 though the inner walls of an old specimen would be quite smooth 

 and without excrescence of any kind. 



This post-embryonic shell is formed both in the Gastropods 

 and Pelecypods of three layers, the outer or " periostracum " 

 being formed almost entirely of " conchyolin," a chitinous sub- 

 stance indissoluble in water, acid, alcohol or ether ; the two- 

 inner layers are composed chiefly of about 95 per cent, of 

 calcium carbonate in the form of calcite or arragortite, the 

 remaining parts being made up of small quantities of calcium 

 phosphate and magnesium carbonate with a small admixture of 

 conchyolin, this compound being known as "ostracum," and, as 

 will be readily seen, is easily subject to erosion by reason of the 

 extreme vulnerability of most of its component parts by acids in 

 the water, hence the outer chitinous layer for its protection. 



The progress of post-larval growth in the Pelecypods is the 

 same, the growth markings appearing as consecutive lines or 

 ridges ; in many of the TJniouidse, and especially the Indian 

 members of the family, the umbonal region of the shell is fre- 

 quently corrugatedly sculptured, this sculpture becoming obsolete 

 and gradually disappearing in the later formed portions of the 

 test, though in some cases, notably of African or Par Eastern 

 forms, the corrugated sculpture is carried on to the end. 



4. Habitat and Mode of Life. The habitat of the freshwater 

 pulmonates and prosobranchs varies usually according to the 

 genus, and even sometimes the species : thus Hydrocena and 

 Cremnoconchus are more or less amphibious dwellers in wet 

 vegetation, clinging to the faces of rocks continually washed by 

 the spray from waterfalls ; Theodoxis and StenotJiyra are in- 

 habitants of either absolutely fresh or brackish water, and even 

 in the case of the former, of pure sea-water, these generally 

 requiring a rocky bottom, while the latter delight in a muddy or 

 sandy bottom on which to crawl ; the habitat of Paludomus is 

 chiefly rocky mountain-streams, though the author has found 

 more than one species plentifully occupying the muddy runlets 

 among the paddy-fields of Ceylon. 



Tiara, Bitliynia, Vivipara, and Pila, as also the pulmonate 

 genera Limncea and Planorbis, chiefly inhabit either stagnant or 

 slow running water; especially where decaying vegetable matter 

 and mud are abundant. 



Among the Pelecypoda the UnionidaB are chiefly, though not 

 always, to be found on sandy bottoms in clear running water, 

 though some species are by no means averse to an abode in the 

 mud of pools and tanks, in which situations they lie almost buried 

 in the sand or mud with only the posterior side and the projecting 

 siphons showing ; Scapliula, which is obviously a descendant from 

 the marine Area, is found not only in the brackish waters of the 

 Gangetic and other Indian deltas but also in perfectly fresh water 



