﻿COSTATiE. 



175 



border ; the costae of the right valve curve slightly downwards, where they are united 

 posteally to the plications upon the marginal carina which is larger than that of the 

 other valve ; the last formed one or two costae in adult specimens are more depressed or 

 become squamous. 



It is doubtful whether the internal mould has been recognised. 



Dimensions of a full sized Cotteswold specimen in the Geological Museum of the 

 University of Oxford. 



Length of the marginal carina . . .27 lines. 



Across the valve at right angles to the carina . . 23 „ 



Convexity of a single valve . . . 9^ ,, 



Greatest breadth of the area . . . . 12 „ 



Dimensions of a full sized Santon specimen in my cabinet. 



Length of the marginal carina . . . 25 lines. 



Across the valve at right angles to the carina . 21 ,, 



Convexity of a single valve . . . 7 „ 



Greatest breadth of the area . . . 9 „ 



History, and local variability. — The specimen first described by me, and figured in 

 the year 1850, was a very young Cotteswold example, and although measuring only three 

 lines across, the valve had about twenty costae, and was supposed to be an adult 

 specimen ; its convexity was so considerable that the figure was almost hemispherical \ 

 larger examples have much less convexity. The hinge-processes and their sulcations are 

 large, they equal in prominence the corresponding parts of any other species of the 

 section having equal general dimensions. The surface ornaments present no well- 

 defined differences between the very young and advanced conditions of growth. A more 

 than common amount of variability occurs in the size and number of the costae, even in 

 specimens from the same locality ; one of our Santon examples, Plate XXXI, fig. 0, 

 having the costae more distantly arranged illustrates this feature, it does not constitute 

 a distinct variety, as it is connected with the typical form by other examples of inter- 

 mediate ribbing ; neither does this variation extend to the ornaments of the area and 

 escutcheon, which are conspicuous for their strongly defined sculpture. 



The largest specimens of T. hemisphcerica have been obtained in the Cotteswold 

 Hills ; it lias occurred only in the middle portion of the range in the district to the east- 

 ward of Cheltenham extending southwards to Stroud and including Rodborough Hill ; 

 from the latter locality all the earlier known specimens were obtained ; its position is 

 the bed termed Lower Trigonia Grit, associated Avith the more abundant T. sculpta and 

 T.formosa, but our species ranks as one of the most rare testacea of the district. 



The museum of the Royal School of Mines has several unusually small examples 

 obtained by the officers of the Survey in the South Lincolnshire district at Stamford, 



