112 Mr. M. 0. March on the 



than lamella I, which is merely a straight ridge. Surely 

 here he is confusing the appearance of teeth and lamellfe. 

 The curving of III is the first stage in the development of 

 3 a and 3 b. Lamella I might appear before lamella III, and 

 yet CA 3 be developed before ]. Indeed, CA 1 may never 

 develop. Also the early appearance of I causes II in the left 

 valve to appear more or less curved, and this enjoins the 

 same fate on III in the right valve. As a matter of fact, 

 this reading of these diagrams supports the assumed change 

 in order of development by showing that, although the first 

 lamella to develop is the most internal, yet the earliest 

 formed teeth appear more externally. 



Order I. PLEURODONTA. 



Pelecypoda in which the prodissoconch stage always shows 

 an embryonic dentition in the form of crenulations, which 

 may or may not continue into the dissoconch stage. The 

 true teeth normally develop as lateral folds at the periphery. 

 The cases in which they do not may be taken as due to 

 acceleration in development. The succession of teeth is 

 normally from external (dorsal) to internal (ventral). The 

 cardinal plateau develops after the first-formed teeth, which 

 mayor may not be traceable from internal ribs. The teeth 

 when developing before the plateau are dysodont, when 

 developing on the plateau they are taxodont. The teeth 

 tend to become curved, so as to lie perpendicular to the 

 hinge-line internally. Peripherally they tend to atrophy. 



Suborder A. Dysodont a. 



Pleurodonta in which the teeth definitely arise as continua- 

 tions of internal ribs. They are reduced in number and may 

 become taxodont in nature or degenerate. The succession 

 may be external (dorsal). 



Suborder B. Taxodont a. 



Pleurodonta in which the origin of teeth from internal 

 ribs is normally not evident. The teeth are numerous and 

 become perpendicular to the hinge-line. The succession is 

 internal (ventral), except where not more than two teeth 

 arise externally (dorsally). 



