32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



Distribution. — The fossil species are from the Eocene and Miocene 

 of California but one Recent form is from off southeastern Australia, 

 and another off the Fiji Islands. 



Discussion. — The name chosen for this genus is unfortunate and 

 inappropriate because the interior details now make it clear that it is 

 totally unrelated to Hemithyris as its name implies. Relationship to 

 the Basiliolidae seems clear in the broad falcifer crura, the details 

 of the deltidial plates, foramen, and smooth exterior. Eohemithyris 

 is the oldest known member of the Basiliolidae but its roots are prob- 

 ably deep in the Cretaceous. It is also interesting that species are living 

 today. 



Hemithyris colurnus Hedley is here assigned to this Eocene genus. 

 This Australian species has never been satisfactorily placed and some 

 objections may be raised to assigning it to Eohemithyris, an Eocene 

 species now known only from California. In spite of the time gap 

 indicated, close comparison of the California and modern Australian 

 species leaves few anatomical points of difference. The exterior of 

 H. colurnus is essentially identical to that of Eohemithyris alexi. 

 Both are thick-shelled forms with translucent to almost transparent 

 shells, especially when they are wet. They are both coarsely fibrous. 

 The beak characters of the two are identical. It is not possible to 

 make a comparison of the pedicle collars of the two species because 

 it is very difficult to determine these details in Eohemithyris. Actually 

 some uncertainty exists as to whether the fossil species has a pedicle 

 collar, but the area of the beak is so thickened that some sort of 

 tubular arrangement must be present. 



Inside the pedicle valve the dental plates of the modern species 

 may be somewhat less prominent than those of the fossil form ; 

 dental plates are definitely present in both however. It is to be ex- 

 pected that those of the older species might be better developed than 

 those of the modern form. The delthyrial cavities and muscle areas 

 of the two seem identical ; the pallial trunks of the modern form are 

 better impressed but this may be a matter of preservation rather than 

 one of generic distinction. 



Inside the brachial valve the crura and hinge plates are almost iden- 

 tical, no features of generic value having been detected. The outer 

 hinge plates are of about the same size, narrower than in Basiliola 

 but much wider than in Aphelesia. The adductor field of the modern 

 species is deeply impressed as in Eohemithyris alexi but the pallial 

 marks of the Recent species are more plainly impressed. The sock- 

 ets of H. colurnus are strongly corrugated but the corrugation of the 



