LOWER HELDERBERG ROCKS. 
207 
&MUS TrEMATOSPIRA (n.g.). 
[ Gr. rprjixa, foramen ; oVeipa, spira .] 
Shell transverse, elliptical or subrliomboidal, ineqiiivalve, furnislied 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 the 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 an 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 opening 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 
costse, 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. 
