METAZOA MOLLUSCA. 197 



The name Pelecypoda is given to the most generalized 

 class of molluscs in preference to Lamellibranchiata for 

 three reasons, viz., it has priority over the latter name ; 

 it is in uniformity with the names of the other classes of 

 Mollusca (Gastropoda, Pteropoda, Cephalopoda) ; the 

 word Lamellibranchiata refers to the gill which is one of 

 the most variable organs of a mollusc, while Pelecypoda, 

 Gastropoda, etc., refer to the foot, one of the most stable 

 molluscan organs. 1 



The classification adopted in this guide is, with certain 

 modifications, that of W. H. Ball 2 and of Dr. Robert T. 

 Jackson. Ball considers the structure and development 

 of the hinge first, and secondly, the sum total of organic 

 characters. As a result of these studies he divides the 

 class into three orders. The first possesses the simplest 

 possible hinge, having the two toothless edges of the 

 shell in contact and united by a ligament. The second 

 has the hinge provided with transverse or cardinal teeth 

 and the third has teeth parallel with the margin and 

 known as lateral teeth. 3 There is no sharp line of divi- 

 sion between the last two orders, as many shells have both 

 the cardinal and the lateral teeth. In these cases the 

 general characters usually enable one to decide to which 

 order a shell belongs. There is here as in every class 

 the difficulty of determining the primitive and the reduced 

 forms. Some shells that do not possess teeth to-day are 

 really the descendants of toothed shell-bearing Mollusca. 

 On the other hand the most primitive Pelecypods were 

 doubtless toothless. Whether these truly primitive forms 

 exist at the present time is a question. Many of the 

 members of Ball's first order have become extremely 



1 Ball, Amer. Journ. Sci., ser. 3, XXX VIII, 1889, p. 446. 



2 Rep. on Mollusca, Bull. Mus. Comp. Zool., XII, 1886; Amer. 

 Journ. Sci., ser. 3, XXXVIII, 1889, p. 445 ; Trans. Wagner Free 

 Inst. Sci., Phila., Ill, part 3, 1895. 



3 See Bernard, Bull. Soc. Ge"ol. de France, ser. 3, XXIII, 1895 ; 

 XXIV, 1896. 



