General Classification of the Felecypoda. 99 



development of teeth in the Taxodonta. The later teeth in 

 the Taxodonta appear veutrally except : — 



(a) la the case of the Pectunculid;o, where the third tooth 

 iu the left valve appears dorsally to the first and 

 second teeth. This may possibly be a case of de- 

 generation^ Bernard, however, does not suggest 

 this(3, p. 61). (Fig. 6.) 



(Z») In Nucula (3, p. 166) two teeth appear dorsally, which 

 Bernard takes as being developed in their normal 

 order. He comments, however, on their irregularity. 

 In the case of the Monomyarians the irregularity iu 

 the development of the dysodont teeth is taken by 

 him as postulating degeneration, so that these dorsal 

 teeth might possii)ly be degenerate. In both these 

 cases the other teeth develop ventrally, 



(c) In the Pectinidae and Spoudylidae, and also in MytUus^ 

 the teeth develop dorsally, 



So far the points established by Bernard are : — 



(1) Til at the Taxodonta [i. e. Prionodesmacea— Naiadacea) 



have an embryonic dentition which is also seen iu 

 some Heterodonta (i.e. Teleodesmacea + Anomalo- 

 desmacea + Naiadacea) . 



(2) That the true taxodont dentition develops parallel to 



the hinge and that its position perpendicular to the 

 cardinal line is due to rotation. 



(3) That the dysodont dentition of the Anisoinyarians is 



an early stage in the development of the taxodont 

 and is originally formed from internal ribs, alter- 

 nating with external ribs when these are present. 



The Development of the Heterodont Dentition. 



The last great point brought out by Bernard is the 

 development of the heterodont teeth. These may or may 

 not show the embryonic crenulations. In either case the 

 true teeth are developed on a common plan. 



The teeth, lateral or cardinal, are developed from laminse 

 running parallel to the edge of the cardinal plateau. Those 

 of the right valve lie ventral to those of the left valve. They 

 are numbered I, II, III, IV, V, VI, from ventral to dorsal, 

 those of the left valve being denoted by the even numbers, 

 and those of the right valve by the odd numbers. As before, 

 there are two sets of these teeth, one lying anterior, and 

 the other postei'ior, to the ligament-pit. 



7* 



