- 1^ 



valve whicli may be iri'-at-are. The curved outline at the juncture 

 of the anterior and dorsal margins, the almost sym-etrieal curve 

 of the base, the numerous narrow radial ribs, the long, narrow 

 crenulations of the inner marr-in are all distinctive. -hese feat- 

 iires, together r;ith the rather high, inflated form, distinguish 

 A. thoniasensis . It ^oould be arlded that the posterior end of the 

 dorsal margin of the specimen is broken off. 



A. agnastha has abo".t the sane number of ribs, but they are wid- 

 er and the shell is more elongate and auricula te. A. \7o rdeni as 

 almost the same shape, but its ribs are much coarser and the ant- 

 erior end of the dorsal marp:-in is sharply angulated. 



Occurrenoe .- Io"-er Miocene: Bowden beds, Bo^rden, Janiaica. 



