﻿GLABRAE. 



97 



angular division ; its surface is smooth, or traversed only by delicate lines of growth. 

 The other portion of the surface has horizontal, broad, depressed, plain, slightly irregular, 

 and unequal ridges, which over the middle and lower portion of the valve become 

 evanescent posteally ; the space thus rendered plain has three or four obscure longitudinal 

 sulcations, and is somewhat more depressed than the costated portion ; its boundary 

 anteally is nearly perpendicular, and extends somewhat anteal to the posteal third of the 

 valve. The umbonal costae are very delicate and closely arranged ; near to the pallial 

 border the costae become widely separated, irregular in their directions, and more 

 obscure. 



The lines of growth are very delicate, they decussate the horizontal anteal extremities 

 of the costae. 



Internally the borders of the valves are smooth, the test is rather thin, and the hinge 

 dental processes have but little prominence. I have no knowledge of the internal 

 mould. 



This fine specimen was obtained by Mr. Vicary, of Exeter, in the Greensand of the 

 Blackdown Hills, near to Collumpton, Devon ; it is shorter posteally than T. longa, Ag., 

 and its few lower costae are more widely separated and irregular. Compared with T. 

 excentrica, Park., the latter has the general figure shorter, more especially anteally ; the 

 convexity is much greater, the umbones are more conspicuous, and the longitudinal 

 ridges are less widely separated. The test generally has greater thickness, and the hinge 

 dental processes project more considerably ; the posteal smooth, wide, depressed space 

 in T. Iceviuscula is also distinctive. 



Mr. Meyer has obtained the species ill preserved in chloritic sandy marl at Duns- 

 comb Cliffs between Beer Head and Sidmouth ; these imperfect specimens and the single 

 example herewith figured are only materials known to me. 



More especially allied to T. Coquandiana, D'Orbigny (' Pal. Fran./ vol. iii, pi. 294,) for 

 which the imperfect specimens first collected were mistaken. It differs from the species 

 of D'Orbigny in the following features : the convexity of the valves is less ; the posterior 

 extremity is shorter or more rounded ; the costae disappear altogether over a considerable 

 portion of the surface posteally ; there is also no indication of the little intercalated rib 

 between each of the rows, as in T. Coquandiana ; the latter species has the costae well 

 defined and passing across the valve continuously its entire length. 



