NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 47 



ters are similar but that genus has well-marked costae and the crura 

 are shorter. 



Geological horizon. — Miocene to Recent. 



Distribution. — All of the northern seas from the Arctic south to 

 Japan in the Pacific and south to the coast of Maine in the Atlantic. 



Assigned species. — The following species, fossil and Recent are 

 placed in Hemithyris: 



Anomia psittacea Gmelin, Pleistocene to Recent, Northern Hemisphere. 

 Hemithyris psittacea alaskana Dall, Recent, Alaska. 

 Rhynchonella zvoodwardi (A. Adams), Recent, Japan. 

 H. braunsi Hayasaka, 1928, Pliocene to Recent, Japan. 

 H. peculiaris Nomura and Hatai, 1936, Recent, Japan. 



Discussion. — The most distinctive features of Hemithyris are the 

 ornamentation and cardinalia. The type species is strongly marked 

 but other species assigned here are very delicately or obscurely orna- 

 mented. Hemithyris is really better described as striate rather than 

 costellate. The surface is marked by radial grooves or impressed 

 lines, fairly uniform in H. psittacea but discontinuous and irregular 

 in H. zvoodzvardi. The spaces intervening between the lines are flat 

 and broad and thus simulate costellae. In H. woodwardi the im- 

 pressed lines are very delicate and so irregular that broad, smooth 

 patches of shell are separated by the striae. These cannot be con- 

 strued as costellae. This type of ornamentation was seen in this 

 study in only one other rhynchonelloid, Plicirhynchia. In this Ar- 

 gentinian genus the region around the umbones is marked as in 

 Hemithyris but the striae do not extend to the costate portion of the 

 valves. 



The crura are unusual because they are long and slender and are 

 usually flattened in a dorsoventral direction rather than laterally as 

 in most of the other genera. They are thus unlike the crura of any 

 other modern rhynchonelloid. The flat blade is attached to the outer 

 hinge plate on its posteroventral surface. In side view the edge of 

 the outer hinge plate forms an oblique ridge and the crus lies at 

 angle under it. The distal end of the crus is usually pointed, the point 

 being on the inside of the plate. This type of crura is generally 

 classified as belonging to the "radulifer" group. 



The cardinal process is seldom conspicuous. It is a triangular area 

 at the apex, roughened horizontally and usually divided by the cleft 

 in the hinge plate which extends to the apex. It is quite like that of 

 several other genera such as Notosaria. 



Median septa or ridges are never well developed in Hemithyris, 



