*Renton limestoKe. 153 



193. 3. TELLINOMYA GIBBOSA (n. *;>.)• 



Pl. XXXIV. Figs. 5a,b. 



Subequilateral, transversely extended to a length more than once and a half the height, 

 compressed below and very gibbous towards the umbones, which are prominent, large and 

 distant, giving a broad space on the cardinal line ; posterior slope oblique ; anterior slope 

 straight, and extended so as to interrupt the curvature of the anterior extremity ; ventral 

 margin with a shallow sinus below the posterior side of the umbones. 



The specimen, though not absolutely denuded of the shell, has the markings entirely 

 obliterated. The form and general character of the surface, and, as far as can be determined, 

 the structure of the shell, correspond with the preceding species of this genus. It differs 

 conspicuously from either of the others, in the more prominent umbones and general 

 gibbosity of the shell, as well as the more nearly equal extremities. The anterior portion 

 of the cardinal slope is more nearly at right angles to a perpendicular drawn from the 

 beak to the base ; thus giving a totally different outline to this part of the shell, when 

 compared with the other species. The slight sinus in the base is a character approaching 

 to T. nasuta J but in this one it is more nearly central, and less conspicuous. 



Fig. 5 a. Right valve of this species, b. Dorsal view. 



Position and locality. In the central part of the Trenton limestone at Middleville. 



194. 4. TELLINOMYA DUBIA ( ti. sp.). 



Pl. XXXIV. Figs. G a, b, c, d, e,f. 



Subequilateral, elliptical, the length almost twice the height, the two extremities nearly 

 equal ; umbones subcentral, moderately elevated, with a scarcely prominent ridge along 

 the posterior slope ; anterior extremity regularly rounded ; posterior one often narrower^ 

 and somewhat obliquely truncated or contracted ; shell thin ; surface marked by fine con- 

 centric lines. The structure of the shell is precisely similar to those already described. 



In this species the umbones are more central than in any other, but the remaining 

 characters appear sufficient to warrant its arrangement with the previous specimens. The 

 posterior extremity is often contracted, though this appears due to accidental circumstances; 

 for in a great number of specimens examined, much the larger proportion were free from 

 any such characteristic. 



Fig. 6 a. Cardinal view of the two valves in connection. 

 Fig. 6 b. Left valve of the same, which is slightly contracted behind. 

 Fig. 6 e. A separate valve, in which the posterior contraction is scarcely perceptible. 

 Fig. 6 d. Left valve of a specimen, in which the posterior side is much contracted. 

 Fig. 6 e, / Separate valve of an old shell, with a view of the dorsal margin. The specimen is almost 

 equilateral, and the two extremities are of nearly corresponding form. 

 I Palaeontology.] 20 



