NO. 5 RHYNCIIONELLOID BRACHIOPODS — COOPER 3 1 



greater depth than the pedicle valve; anterior commissure uniplicate, 

 the fold being broad and gentle ; beak erect, small, inconspicuous ; 

 foramen small, round, slightly auriculate, submesothyrid ; deltidial 

 plates conjunct. Surface smooth but with obscure anterior costation 

 in old individuals. 



Pedicle valve with narrow delthyrial cavity bounded by short dental 

 plates plastered against the shell wall ; vascula media short, prominent. 



Brachial valve with short, slender socket ridges bounding narrow 

 corrugated sockets ; outer socket plates moderately broad and shallow, 

 attached to long and exceptionally broad falcifer crura ; adductor field 

 subcircular, deeply inserted ; pallial trunks not impressed. 



Type species (by original designation). — Eohemithiris alexi Hert- 

 lein and Grant, 1944, p. 55. 



Comparison. — The entire anatomy of Eohcmithyris is most like 

 that of Basiliola and other members of the Basiliolidae. It differs, 

 however, in being more nearly equivalve, whereas Basiliola is strongly 

 inequivalve ; the deltidium of Eohcmithyris is only slightly auriculate 

 and does not have the elaborate development of the pedicle collar 

 seen in Basiliola. The pallial marks of Basiliola are more strongly 

 developed in all species but those of the type species of Eohcmithyris 

 are much abbreviated. The outer hinge plates attaching the crura to 

 the socket ridges are narrower in Eohcmithyris than in Basiliola. 



Comparison of Eohcmithyris and Rhytirhynchia is essentially the 

 same as that with Basiliola. The brachial valve of Rhytirhynchia is 

 much deeper than that of Eohcmithyris but the anterior costation of 

 the former is much stronger than that seen in Eohcmithyris which 

 seems to be a rare feature. 



Eohemithyris is quite suggestive of Aetheia in outline and beak 

 characters but differs from it in interior details. No development of 

 inner hinge plates appears to have taken place in Eohemithyris. 



No other members of the Basiliolidae compare closely with Eohemi- 

 thyris. 



Geological horizon. — Eocene (Domengine and Capay formations). 



Assigned species. — Four species are assigned to this genus, two 

 fossil and two Recent : 



Eohemithiris alexi Hertlein and Grant, Eocene, California. 

 Eohemithyris? gettysbitrgensis Cooper, Miocene, California. 

 Hemiihyris columns Hedley, Recent, Australia. 

 Rhynchonella grayi Woodward, Recent, Fiji Islands. 



was thought by its authors to be similar to Hemithyris and an early relative of 

 it, the correction of spelling in the latter by Bronn is essential in the former. 

 The spelling of Eohemithiris is corrected to Eohemithyris in Zoological Record 

 for 1950, p. 21. 



