6 March, j\Iorphogenesis of certain Pelecypoda. 



Classification as based on Ornament. 



The forms here described fall under Lycett's classifica- 

 tion into two groups, 



{a) The Undulatse, comprising the Inferior Oolite to 

 Corallian forms. 



{b) The Gibbos^e, a sub-group of the Glabra?, in- 

 cluding the Portlandian forms. [Bigot.] 



It is, I think, permissible to class these two together 

 in one great sub-division, the Undulatae. 



The more usual ancestry for the Gibbosas is to 

 describe them as degenerate Clavellates. The reasons 

 given for doing this are : — 



(i) Both Clavellates and Gibbosai have concentrically 

 marked areas. 



(2) The Clavellate umbones are concentrically marked, 



the Gibbosse type being derived from the 

 Clavellate by loss of the clavellations. 



(3) The ancestry of the GibbosiK is unknown, and, 



although the umbones resemble M. ciitvirostris^ 

 yet it is impossible to get a Triassic type in 

 the Portlandian without any intervening links. 



Against this theory and for the junction of Undulat^u 

 [Lye] and Gibbosje may be said, 



(i) That there is a fairly complete ancestry bridging 

 over the gap between Triassic and Portlandian 

 forms. 



(2) That the Undulatae agree with the Gibbosae in 



umbonal markings in both flank and area. 



(3) That the Clavellate umbonal flank markings, as 



I hope to show later, are not concentric. 



