LOWER HELDERBERG ROCKS. 211 



Trematospira simplex ( n. s). 



Plate XXVIII A. Fig. 2 a -/. 



Shell transversely elliptical or subrhomboidal, nearly once and a half as 

 wide as long : hinge line scarcely declining from the beaks ; cardinal 

 extremities rounded : valves moderately and almost equally convex, 

 closely compressed at the latero-oardinal margins. Ventral valve having 

 a well defined mesial sinus, which extends more than half way from 

 beak to base : beak elevated, scarcely incurved, and vertically trun- 

 cated by a small perforation, between which and the hinge line is a 

 well-defined deltidium. Dorsal valve moderately convex in the middle, 

 and slightly elevated towards the front of the shell : beak closely 

 incurved beneath the deltidium of the opposite valve. 



Surface marked by four or five simple plications on each side of the 

 centre, two of which on the ventral valve are involved in the sinus ; 

 and three upon the opposite valve, the central one being broader and 

 flattened towards the base : entire surface granulose or punctate, with 

 faint indications of growth lines. 



Trematospira simplex, var. 



A larger imperfect specimen, apparently of the same species, has the 

 beak slightly more incurved, the mesial plication depressed in the centre, 

 and those on each side bifurcating near the base, as are those which 

 border the sinus of the ventral valve. The mesial sinus in this specimen 

 is much deeper, and the front of the opposite valve much more elevated. 



This species, in its general form, resembles the T. camura of the Niagara group; 

 but the shell is somewhat more robust, and the plications are less numerous and 

 stronger. The Niagara species has two smaller plications forming the mesial fold, 

 which often become obsolete before reaching the beak; and the sinus of the ventral 

 valve is often marked by one or two smaller plications, which sometimes extend 

 more than half way to the beak. It differs also from this species in the strong lines 

 of growth, which are seen in many of the specimens. These characters are shown 

 In the figures 3 k, I, m, n of Plate lvi, Vol. ii, Palaeontology 'of New-Tork. 



