LOWER HELDERBERG ROCKS. a07 



CfEHfllS TREMATOSPIRA (n.g.). 

 [ Gr. TpjifMt, foramen ; (fidtpa, spira.] 



Shell transverse, elliptical or subrhomboidal, inequivalve, furnished with 

 internal spires (arranged as in Spirifer). Hinge line shorter than the 

 width of the shell : cardinal angles rounded. Valves articulated by 

 teeth and sockets : beak of ventral valve produced or incurved, and 

 truncated by a small round perforation, separated from the hinge line 

 by a deltidium. A deep triangular pit, or foramen, beneath the beak 

 of the ventral valve, which is filled by the closely incurved beak of 

 the dorsal valve. False area sometimes defined. 



Surface marked either with strong simple plications or finer fasciculate 

 or bifurcating striae, which cover also the mesial elevation and de- 

 pression. Shell structure punctate ? 



In the extension of the hinge line, the mesial sinus, the internal spires, and, 

 partially, in tlie exterior markings, this genus resembles Spirifer and Spiriferina. 

 In the perforate beak, false area, and incurvature of the beak of the dorsal valve 

 beneath the apex of the opposite valve, it resembles Atrypa; while one of the 

 species has the general aspect of Rhynchonella. From Retzia, Spirigera, and 

 Merista, which have similar internal spires, it is separated by external and other 

 important characters. The appearance of an area is deceptive; depending mainly 

 upon a partial displacement of the valves, which presents to view the hinge line of 

 the ventral valve. This is true of T. multistriata, where we find some specimens 

 with ail appearance of an area, and others without. The T. perforata is clearly 

 without an area, as well as T. costata; though the margin of the foramen in the 

 former of these is often defined in such a manner as to resemble a true area. The 

 broad triangular foramen or pit for the reception of the beak of the dorsal valve is 

 a constant and conspicuous feature. This pit does not appear to be like the foramen 

 of Spirifer, an oi)ening into the cavity of the valve, but is spoonshaped, somewhat 

 like that of Pentamerus; its lateral walls in T. multistriata having been traced for 

 some distance below the margins, apparently converging towards each other. 



The known species present the variety of surface marking, respectively, of simple 

 costae, strong angular strise in fascicles, and finer somewhat rounded bifurcating or 

 simple strise. Specimens of each are rare; and of T. costata, but a single one was 

 found among collections continued uninterruptedly through a period of ten years. 



