8 March, Morphogenesis of certain Pelecypoda. 



differences. The variation in ornament, if worked out 

 with its relation to shape, may show relationships of forms 

 whose positions have hitherto been considered doubtful. 



Extreme Types of Ornament. 

 (li) T. irregularis. This is an elongated Kimmeridgian 

 and Oxfordian form. The extreme deviation of the 



Fig, ia. 



Showing the apparent lines of ornament on T. inegtdaris. 



/ /l 



Fig. ']b. 

 Showing the true lines of ornament on T. iyrcgularis. 



posterior radials and the well-marked uplift of the anterior 

 concentrics result in the marked bending and even break- 

 ing of tl)e lines of ornament. 



