2 March, Morphogenesis of certain Pelecypoda. 



strongly toothed to totally edentulous forms. So far as 

 has yet been worked out the Trigoniae show a marked 

 uniformity in teeth, muscle scars, and pallial line, the most 

 marked variation being the development of ligamental 

 pits in the Scabrae (Lycett). Some other method of 

 classification must, therefore, be used in their case. This 

 is obviously the development of their ornament. 



Development of Coficentric Trigonid Ornament. 



The Trigoniai are definitely known from the Trias, 

 so that it might be inferred that in Triassic times the 

 ornament would not have diverged from the simplest {i.e., 

 concentric) type. But this is not the case. Three types of 

 ornament are known in Triassic times. 



{a) Pure concentric. Myophoria curvirostris. 



{b) Concentric, showing the introduction of tuber- 

 culate radial on the posterior part of the area. M. lineata. 

 {Figs. I. and II. of Plate) 



[c) Tuberculate radial. T. harpa. 



Since no intermediate types are known, it seems 

 probable that the Trigonise are either polygenetic, or 

 that their origin is so far pre-Triassic as to allow of the 

 development of two of the three m.ain types, a«nd extinction 

 of the intermediate forms of ornament by Triassic times. 



However that may be, it should be possible to trace 

 out the subsequent development of these types of orna- 

 ment, and the first to be followed is naturally the con- 

 centric. 



In the Middle Lias occurs T. lingonensis. This is 

 purely concentric in ornament on both area and flank. 

 Its later stages, however, tend to become smooth. Its 

 importance, therefore, does not lie in its being in the main 



