﻿COSTAM. 



149 



In common with other examples of the Costata the marginal carina of the right 

 valve is much larger than that of the other ; it overwraps and partly conceals the post- 

 carinal groove. 



The other portion of the shell has about twenty-four large, plain costse, all of which 

 originate at the anterior border; they are small and delicate, their borders are indented or 

 rendered nodulous by oblique, decussating lines of growth, which are conspicuous upon the 

 anteal portion of the shell. At the curvature of the valve in passing to the side the costae 

 form a considerable downward curvature ; they become horizontal about the middle of the 

 valve, and form a second slight downward curvature as they approach the marginal carina, 

 to which their extremities are united in the right valve, but the costae of the other valve 

 are separated from the carina, their extremities terminating abruptly at the well-defined 

 ante-carinal groove. 



Examples of the very young shell, when only four or five lines in length, have the 

 anteal truncation less well defined ; the three carinas upon the area are prominent, acute, 

 and without indentations ; the intercarinal costellae are scarcely formed, or there is a 

 single small costella in each of the intercarinal spaces. 

 Comparative measurements of the two varieties: 



C Diameter through the united valves 2f inches. 

 The typical form 3 Length upon the marginal carina inches. 



/ Across the valve at right angles to the carina 2-^- inches. 



C Diameter through the united valves 1^- inches. 

 Variety lata J Length upon the marginal carina 2^% inches. 



/ Across the valve at right angles to the carina 2- x % inches. 



A good figure of the left valve representing the typical form is given by Agassiz 

 (' Trigonies,' tab. iii, fig. 12), but figure 14, which is intended as a delineation of the area 

 of the rigid valve, has apparently been drawn by the aid of a looking-glass from a 

 specimen of the left valve, and is consequently altogether incorrect. 



An excellent figure of the right valve is given by Quenstedt (' Der Jura/ p. 502). 



Positions and Localities. Both varieties of T. costata occur together in beds of 

 Inferior Oolite at various localities in the south-western counties, as at Bradford Abbas, 

 from whence good illustrative specimens have been kindly forwarded to me by Professor 

 Buckman ; other well-known localities are Burton Bradstock, Chideock, Half-way House 

 Quarry, Yeovil, Dundry, &c. Throughout the range of the Cotteswold Hills one or both 

 of its varieties occur at many places, but apparently only over small areas ; the external 

 casts are sometimes clustered in great profusion in the bed called Upper Trigonia-grit, 

 but good specimens with the tests preserved are comparatively rare. 



In the extension of the same formation through Oxfordshire, Northamptonshire, 

 Rutlandshire, and Southern Lincolnshire, the species is comparatively rare, and in 

 Northern Lincolnshire it is absent. 



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