NO. 5 RHYNCHONELLOID BRACHIOPODS — COOPER 57 



Joubin, Resultats voyage S. Y. Belgica, 1897- 1898- 1899, Zool., Rapt. 

 Sci. Commiss. Belgica, p. 5, pi. 1, figs. 1-4, 1901. 



Comparisons. — The genera to which Compsothyris can profitably 

 be compared are: Frieleia, Hispanirhynchia, and Grammetaria. The 

 differences between Compsothyris and Frieleia are chiefly exterior 

 characters but the cardinalia also vary importantly. Frieleia is not 

 so strongly and evenly costellate as Compsothyris and the shape and 

 folding of the two are quite different. In Frieleia it is common for 

 both valves to have a sulcus and for the front to be emarginate. 

 Compsothyris is faintly uniplicate. Furthermore, Frieleia is much 

 more triangular than Compsothyris and has a great development of 

 inner hinge plates in the cardinalia of the brachial valve, a feature 

 not shared by the Antarctic shell. 



Hispanirhynchia differs in shape from Compsothyris, having a 

 compressed profile and strongly triangular outline. The two genera 

 are similarly marked on the exterior however. Inside the brachial 

 valve only a slight development of inner hinge plates appears in 

 Compsothyris, and the small chamber at the apex is not obliterated by 

 shell growth in the adults as it is in Hispanirhynchia. 



Compsothyris differs from Grammetaria in the form of the valves 

 and in the less elaborate deltidial plates. Compsothyris is nearly 

 circular in outline whereas Grammetaria is strongly triangular. The 

 deltidial plates of Grammetaria are elaborately auriculate but those of 

 the Antarctic genus are small and not auriculated. Inside the brachial 

 valve no inner hinge plates are developed by Grammetaria but the 

 small apical chamber present in the young is sealed off in the adult 

 by the sides growing shut. In adult Compsothyris this chamber 

 remains open. 



Geological horizon. — Not known in the fossil state. 



Distribution. — Ross Sea area and western Antarctic in depths 

 ranging from 45 to 300 fathoms. 



Assigned species. — So far only one species can be definitely as- 

 signed here but two others may belong: 



Rhynhonella racovitsae Joubin, Recent, Antarctic. 

 ? Hemithyris striata Thomson, Recent, Antarctic. 

 ? Rhynchonclla valdiviae Helmcke, Recent, Indian Ocean. 



Discussion. — Jackson (1918, p. 193) expressed interest over the 

 fact that the features of Compsothyris and Frieleia were suggestive 

 of certain Paleozoic genera, especially Camarotoechia. Comparison 

 with interiors of Camarotoechia (see pi. 4, D, figs. 6-8), however, 

 show the relationship to be quite remote because the structures in 



