General Class'ijication of the Pelecypoda. 



95 



of alternating ridges and folds, called by liim " crcuulations." 

 This baud is separated into anterior and posterior portions 

 by the primary ligamental pit. In Ostrea, however, the 

 anterior row is wanting, and the ligamental pit lies at the 

 anterior edge of the shell. Subsequent to the development 

 of the crenulations the true teeth make their appearance. 

 These arise, not perpendicular to the hinge-line but as long 

 ridges parallel to it. They may retain this position through- 

 out life, as in Cucullea crassatina. Usually, however, the 

 interior end becomes sharply curved and the external part 

 atrophies, leaving the usual taxodout teeth (figs. 1, 2, and 3). 



Young CucuUea crassatina, showing the recurving of the primitive 

 lamellae to form taxodont teeth. (After Bernard.) 



Li = primary ligamental teeth. 



The great importance of this is twofold : — 



Firstly. It refutes the theory that the early embryonic 

 dentition seen in certain Heterodonts, and which 

 arises perpendicular to the hinge-line, represents an 

 early taxodont condition, and for that reason necessi- 

 tates the descent of the Heterodonts from Taxo- 

 donts as seen in modern forms. 



Secondly. It does away with the radical difference between 

 the heterodont and taxodont teeth. 



The Origin of the Pleurodont Dentition. 



Amongst the Anisomyarians (figs. 4 & 5) the teeth show 

 a still earlier stage than in the Nuculidae and their allies. 

 Here they rise before the development of the cardinal plateau 

 as lateral folds alternating with the external ribs. These 

 internal ribs may occur where the outer test is smooth. In 

 forms where the test thickens greatly a transition can be 



