NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 35 



commissure deeply sulcate; surface smooth. Beak of pedicle valve 

 short, nearly straight, and bluntly pointed; foramen of moderate size, 

 hypothyrid ; deltidial plates disjunct. 



Pedicle valve interior with small corrugated teeth supported by 

 short dental plates which define a small delthyrial chamber ; muscle 

 area small. 



Brachial valve interior with corrugated sockets bounded by over- 

 hanging socket ridges ; crura short, falcifer type, crescentic in section, 

 attached to socket ridges by broad outer hinge plates. Inner hinge 

 plates small and inconspicuous. Median ridge short and reaching the 

 apex. Adductor field small. 



Type species (by original designation). — Hemithyris strebeli Dall, 

 Bull. Mus. Comp. Zool., Harvard Coll., vol. 43, p. 441, 1908. 



Comparison and discussion. — The important and striking difference 

 between Neorhynchia and all known Recent and Tertiary rhynchonel- 

 loid genera is the sulcation of the anterior commissure. Rhynchonel- 

 loids of similar habit are known from the Mesozoic. They are also 

 known from the Devonian, Mississippian, and Permian as well. 

 Sulcation is a folding tendency that has appeared many times in dif- 

 ferent stocks of the rhynchonelloids. 



Neorhynchia is most closely related to Basiliola in the presence of 

 the wide outer hinge plates and falcifer crura having a crescentic sec- 

 tion. The presence of incipient inner hinge plates in Neorhynchia is 

 another difference between the two genera. 



Assigned species. — Only one species is so far known in this genus : 

 Hemithyris strebeli Dall, Recent, Pacific. 



Distribution. — The known specimens of this species are all from 

 great depths: 2,084 fathoms at 35.i°F. in mid-Pacific and 2,035 

 fathoms at 35.3 ° F. off the Galapagos Islands, both on Globigerina 

 ooze. 



Genus RHYTIRHYNCHIA Cooper, 1957 

 Plate 11, A 

 Rhytirhynchia Cooper, U. S. Geol. Surv. Prof. Pap. 314-A, p. 8, 1957. 



Subcircular to suboval in outline and with the maximum width at 

 midvalve; strongly inequivalve, the brachial valve being swollen and 

 deep ; anterior commissure sulciplicate, surface smooth except ante- 

 rior which is paucicostate. Beak small, rounded, inconspicuous ; fora- 

 men rounded, submesothyrid to mesothyrid; deltidial plates short, 

 conjunct. 



Pedicle valve with thick, coarsely corrugated teeth and moderately 

 developed to remnantal dental plates ; pedicle collar well formed ; 



