﻿COSTAT^. 159 



Hill, and the Inferior Oolite of Oxfordshire and Northamptonshire ; Mr. Sharp has also 

 collected it in the Lincolnshire Limestone of Tinkler's Quarry near Shamford. 



A distinct and smaller variety {Chelfensis) occurs in the Cotteswold Hills to the 

 eastward of Cheltenham ; the general outline agrees with the typical form, but the valves 

 have somewhat less convexity and are less massive ; the costse are much smaller and 

 more closely arranged ; the area and escutheon possess all the strongly marked characters 

 which distinguish the species. Plate XXXIV, figure 3, exemplifies this variety. 



A variety designated Eolandi in Mr. Cross's memoir, Plate XXXIV, fig. 4, must also 

 be arranged with T. sculpta ; it appears to be limited stratigraphically to the upper 

 division of the Great Oolite formation, including the Forest-marble and Cornbrash. It 

 was figured by the venerated author of " Strata Identified," at p. 05 of that work, as 

 a characteristic fossil of the Cornbrash. Its surface-ornaments agree closely with those of 

 the typical form, from which it differs in the lesser breadth of the costated portion of the 

 valve, so that the general figure is shorter, and the area, which is very wide, occupies a 

 much larger proportion of the sm-face ; the carinse and intermediate costellap, with their 

 denticulations, possess all the prominence which characterises the two other varieties, and 

 these features are conspicuous even in the smaller specimens. Our figure, Plate XXXIV, 

 fig. 4, represents a specimen of medium size. This variety has occurred at several 

 localities in Wiltshire, Oxfordshire, and Northamptonshire, and also at Appleby, North- 

 Western Lincolnshire ; it appears to be somewhat rare. 



Affinities. — The Lyriodon simile of Bronn (' Letha3a,' tab. xx, fig. 3), afterwards figured 

 by Agassiz under the name of Trigonia similis (' Trigon.,' tab. ii, figs. 18 — 21), also by 

 Quenstedt ('Der Jura,' tab, xlv, fig. 15) under the name of T. co5/«^a, has affinities with 

 T. sculpta in the general figure of the shell and more especially in the costse ; the coarsely 

 sculptured area also possesses some resemblance. It differs in the general uniformity 

 of the area, which is almost destitute of a median carina, and in the much greater angle 

 which the surface of the area forms with the other portion of the siu-face, from the lesser 

 convexity of the shell ; the dental processes are also smaller and less massive, occupying 

 a smaller portion of the interior of the shell. 



The Australian T. Moorei, Lye, figured with the description of T. costata (p. 151), 

 resembles T. sculpta in the general figure and in the costte ; the surface-ornaments 

 of the area , including the carin8e, are, however, much less prominent, the escutcheon is 

 much narrower, imparting a greater depression to that portion of the shell. 



The figures named T. costata in the ' Versteinerungen ' of Knorr and in the 

 ' Encyclopedic Methodique,' appear to have been drawn from specimens of T. sculpta ; they 

 are very coarsely engraved, and the surface-ornaments present features apparently much 

 exaggerated, even when compared with the deeply indented sculpture of that species. 



