PELECTPODA 



OF THE 



CEETACEOUS ROCKS OP SOUTHERN INDIA. 



Sub-ldngdom, MOLLUSCA. 



Class, PELEOYPODA, Qoldfuss. 



Characters. — Bilateral, aquatic Mollusca, with the mouth and anus situated 

 at opposite, or nearly opposite, ends, without a distinct head, or any solid buccal 

 plates in the mouth; breathing through partially free lamelliform gills, attached at the 

 sides of the body ; effecting their movements by an elongated compressed foot, situated 

 ventrally ; enveloped %n a mantle which is dorsally always united, in front and 

 behind, and often also ventrally, separated; secreting laterally two equator sub-equal 

 solid valves which partially, or wholly, cover the fleshy part of the animal, the 

 latter being connected with the valves by one or two strong retractile muscles, while 

 the valves themselves are along the dorsal side joined to each other by more or less 

 distinctly developed hinge-teeth and internal, or external, horny ligaments. 



As I shall often have occasion to allude to the importance of the anatomical 

 characters in the classification of the different orders and families, I will first give 

 a brief general explanatory account of the principal points in the organisation 

 of the class Pelecypoda, and then I will state my reason for having adopted 

 the latter somewhat uncommon denomination. 



The form of the body is, with very few exceptions (and these occur in every 

 class), elongated or roundly oval, surrounded by a fleshy mantle, the external 

 edges of which are always partially free and secrete laterally two perlaceous or 

 calcareous valves. The principal variations in the form of the body are — 



\st, — Eorms represented by the well known ship-worms, in which the posterior 

 part of the animal is greatly prolonged, and secretes a more or less continuous 

 calcareous tube which in some cases encloses the rudimentary valves of the species, 

 in others becomes partially or entirely confluent with them. This elongated form 

 of all the so-called FROLABACEA, which bore in wood, rock or sand, &c., is 

 essentially connected with their habitat. 



2nd, — The body is oval or rounded, with two free, equal, or sub-equal, but 

 quite similarly formed valves. To this group by far the largest number of 



