Man Chester Memoirs, Vol. Iv. (191 1), A^^. 11. 



XI. Studies in the Morphogenesis of certain Pelecypoda. 

 (2) The Ancestry of Trigonia gibbosa. 



By Margaret Collev March, B.Sc. 



(Comviunicated by Dr. George Hickling.) 

 Read February jfh, rgii. Received for publication Marcli jik, igii. 



The Trigoniffi were subdivided by Lycett into eight 

 groups. This classification was made in 1875, ^"^ was, 

 therefore, based on adult shell characters. The subsequent 

 work of Hyatt, Beecher, and Jackson has shown that the 

 characters of true taxonomic value are nepionic. In 

 trying to construct a classi ication of the Trigoniae, there- 

 fore, the characters to note are those of the early shell. 



The classification of other fossil Pelecypoda has been 

 worked out along two lines, either on form or shell 

 structure. 



Alteration in form was used by Jackson in his 

 classification of the Aviculida^. In this case the group 

 showed progressive adaptation to the sedentary habit, so 

 the modification in form was biological. The Trigoniae, 

 however, show no such progressive adaptation, — their 

 modification, therefore, is of a purely individual character 

 and dependent on environment. Such purely cecological 

 variation is of no taxonomic value. 



Alteration in teeth structure, muscle scars, and pallial 

 line has been used as the basis for a general classification. 

 Variation in teeth can be used in the classification of the 

 Unionidge, which show perfectly graded series from 



April 20th, igii. 



