﻿COSTAT^. 163 



large, prominent, nearly straight, with deeply indented plications throughout its length. 

 The other portion of the shell has the costse (28 or 29) moderately elevated, narrow at 

 their upper borders, separated by wider spaces, very oblique in their direction, and have 

 little curvature ; when the upper border of a valve is placed in a horizontal position, the 

 costaj have their general direction nearly parallel with it, excepting near to the anterior 

 border, where they are attenuated and curve upwards ; they are therefore without the 

 anteal undulation seen in T. cosiata, T. tenuicosta, and others of the same section ; 

 posteriorly they terminate abruptly at the strongly defined antecarinal groove of the left 

 valve; they are united to the carina of the other valve. The lines of growth are 

 conspicuous ; they decussate and indent the anteal portions of the costse. 



Affinities and Differences. To separate it from T. costata it is only necessary to 

 compare the general figure and proportions of the several parts of the valves above 

 described, which will be found to be altogether dissimilar ; the escutcheon, small, 

 excavated, with very delicate costellae, would alone be sufficient to exemplify its 

 distinctness. 



T. ienvicosta, another allied species, has the umbones much more narrow and produced ; 

 the general convexity is greater, the escutcheon is much wider, the area is m.ore excavated, 

 and its carinas are small in conformity mth the very delicate intercarinal costellae. From 

 others of the costata, the large area, the unusual prominence of the median carina, and 

 the great length of the siphonal border afford differences sulficiently conspicuous. 

 T. carinata alone has the costse more oblique, but in other respects is only remotely 

 allied to it. 



Dimensions of two specimens in my cabinet : 

 Length upon the marginal carina 

 Across the valves at right angles to the carina 

 Breadth of the area 

 Length of the escutcheon 

 Breadth of the escutcheon in the united valves 

 Length of the siphonal border 

 Convexity of a single valve 



Position and Locality. This well-characterised and remarkable species of the Costatce 

 has been hitherto obtained only in the ferruginous pisolite or Cephalopod-bed at Bradford 

 Abbas. Apparently T. hella is somewhat rare ; and, like its congeneric associate, 

 T. striata, it has not been collected to the northward of the Carboniferous rocks of the 

 Bristol coal-field. I have been favoured with specimens by Colonel jMansel Pleydell and 

 'by Professor Buckman ; the examples figured are not of the largest dimensions. 



