General Classificition of the Pelecypoda. 99 



development of teeth in the Taxodonta. Tlie later teeth in 

 the Taxodonta appear veutrally except : — 



(a) In the case of the Pectunculid;e, where the third tooth 



in the left valve appears dqrsally to the first and 

 second teeth. This may possihly be a case qf de- 

 generation^ Bernard, however, does not suggest 

 this (3, p. 61). (F.g.6.) 



(b) In Niicula (3, p. 166j 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 in 

 the development of the dysodont teeth is taken by 

 him as postulating degeneration, so that these dorsal 

 teeth might possibly be degenerate. In both these 

 cases the other teeth develop ventrally. 



(c) In the Pectiuidse and S[)oiidylida8, and also in Mytiliis, 



the teeth develop dorsq,lly, 



So far the points established by Bernard are : — 



(1) That the Taxodonta {i, e. Prionodesmacea — Naiadacea) 



have an embryonic dentition whicii is also seen in 

 some Heterodonta [i, e. Teleodesmacea + Auomalo- 

 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 Anisomyarians is 



an early stage in the development of the taxodont 

 and is origing,lly 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 tq 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 numbe^7s, 

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

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

 the qther posterior, to the ligament-pit. 



, 7* 



