General Classification of the Pelecypoda. 



95 



of alternating ridges and folds, called by liini " crcnulations." 

 This band is separated into anterior and posterior portions 

 by the primary liganicntal pit. In Ostren, howcvcr_, the 

 anterior row is wanting, and tlie iigamental 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. Tiiey may retain this position throagli- 

 out life, as in CucuUea crassalina. Usually, however, the 

 interior end becomes sharply curved and the external part 

 atrophies, leaving the usual taxodont teeth (figs. 1, 2, and .'5j. 



Fig. I 



Young CiiaiUea crn^xrttina, showing the recurving of the primitive 

 hiniellte to form taxodont teeth. (After Bernard.) 



Iji = primary Iigamental teeth. 



The great importance of this is twofold : — 



Firsthj. 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. 



Secondhj. It does away with the radical difference between 

 the heterodont and taxodont teeth. 



77^6 Origin of the Pleurodont Dentition. 



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

 a still earlier stage than in the Nuculidre 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 Ki'catlv a transition can be 



