OP SOUTHEEN INDIA. 7 



l^e turned perpendicularly downwards, a position which is actually only to be 

 naturally observed in many MYA O^A, but rarely in any of the other orders. The 

 terms anterior and poste7'ior declivity are identical with lunular and areal slope, 

 and are restricted to the dorsal, generally convex, region of the animal and shell. 



The valves composing the shell either fit closely with their margins toge- 

 ther, or they form a gape anteriorly, or posteriorly, or ventrally, mostly at the 

 place where the foot protrudes ; the valves are consequently termed either closed 

 or gaping. The distance from the anterior end to the posterior will be spoken 

 of as the length of the shell, and in the measurements given the extreme length 

 will be always recorded. A line drawn from the top of the umbo perpendicu- 

 larly upon the length and extending to the opposite ventral margin will be 

 understood as the height; and the shortest straight line between the greatest 

 convexities of the valves will be noted as thickness. The terms right and left 

 valve are self-understood from the above noted position. The side of the shell 

 where the mouth of the animal is situated and nearer to which, as a rule, the 

 foot protrudes is the anterior, it is in a greater number of Pelecypoda also the 

 shorter one. The only slightly confusing difficulty arises in case of the shells livino^ 

 attached, their form being then often greatly altered ; but even as regards these I 

 shall retain the same uniform terminology, placing them always in such a position 

 that the umbones are situated above, and the margins of the lunular and areal 

 declivities either in a horizontal line or sloping under similar angles, as the case 

 may be. The terms attached, or larger, and free, or smaller, valve may often be 

 conveniently used, without in any way interfering with the above terminology. 



The expressions of cardinal and lateral teeth, external ligament, fulcra or 

 nymphae, internal ligament or cartilage, cartilage processes, or pits, pallial and 

 muscular impressions, &c., are so commonly and uniformly used that I do not 

 need relate them here in detail. 



The extent to which the mantle-margins are united or separated, the form 

 and number of the retractile muscles, the shape of the labial palps, gills, siphons, 

 and of the foot, together with the general form and structure of the shells, the 

 position of the ligament and cartilage, and the form, number, and disposition of the 

 hinge-teeth, supply the principal distinctions upon which our classification into 

 orders and families is chiefly based. 



I now return to the question concerning the propriety of the term Pelecypoda, 

 which I have here adopted for this class of Mollusca. It is essential that this 

 point should be discussed upon all its merits. 



One of the oldest names applied to this class is that of dithyra, first used 

 for it by Aristotle. It was very early (1681) changed into bivalvia, and by 

 Lamarck (1818) into conchifera. Both the latter names became very popular 

 among conchologists, while Turton and Swainson again introduced the original 

 appellation of Aristotle, though not with success. 



Blainville, having regard to the lamellar shape of the gills, proposed (1814) 

 the name lamellibrancsiata; sometime afterwards (1828) the more correctly 



