4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



workers. Yabe and Hatai (1934), for example, distinguished their 

 genus Neohemithyris (= Basiliola) from Hemithyris chiefly on this 

 basis. 



Another feature given importance in the study of the rhynchonel- 

 loids is the position of the foramen in relation to the beak. A common 

 condition is one in which the entire foramen is surrounded by the del- 

 tidial plates and is called a hypothyrid foramen. In other genera the 

 foramen has migrated posteriorly because of pedicle pressure and has 

 thus resorbed or worn away the portion of the deltidial plates on its 

 posterior side. In this condition, which is called submesothyrid, the 

 foramen is bounded posteriorly by the beak and anteriorly by the del- 

 tidial plates. A further condition is called mesothyrid and results from 

 continued posterior migration of the foramen, which has resorbed part 

 of the beak and is bounded posteriorly by part of the curving umbo and 

 the deltidial plates anteriorly. This condition is rare in modern and 

 Tertiary rhynchonelloids. 



One of the most characteristic rhynchonelloid features is the rims 

 or winglike extensions that adorn the deltidial plates of some genera. 

 Perhaps the most exaggerated modern examples are those of Gram- 

 metaria and some species of Cryptopora in which the deltidial plates 

 bear prominent lateral extensions. The more common condition is that 

 of Basiliola in which the lateral and anterior margins of the deltidial 

 plates in contact with the foramen are reflected dorsally in the direc- 

 tion of the brachial valve and form a conspicuous lip around the fora- 

 men. This may have helped, in conjunction with the pedicle collar, 

 to form a tube which strengthened the hold of the valve on the pedicle. 



INTERIOR CHARACTERS OF THE PEDICLE VALVE 



Most of the Recent and Tertiary rhynchonelloids have the beak and 

 pedicle regions strengthened by a pedicle collar. An elaborate collar 

 is developed in Basiliola. The deltidial plates are conjunct and auricu- 

 late. Their inner margin grows laterally along the sides of the del- 

 thyrial cavity to meet on the floor of the valve. In B. pompholyx 

 (U.S.N.M. 274135) the anterior margin of the collar protrudes an- 

 terior to the edge of the deltidial plates and is elevated above the 

 valve floor (pi. 12, figs. 9, 10). In B. beecheri the anterior ends of 

 the deltidial plates are thickened and expanded inward to form a flat 

 area that rides over the umbo of the brachial valve when the valves 

 are opened and closed (pi. 14, A, fig. 2). 



Hemithyris possesses a pedicle collar but it is not complete because 

 the deltidial plates are disjunct. The inner edges of the deltidial plates 



