﻿SCABRtE. 131 



undoubtedly nearly allied ; more especially small and immature examples of each are 

 often difficult to distinguish. Adult forms present some differences of figure ; 1\ 

 scabricola is more inflated anteally, and its anterior face is more flattened ; the umbones 

 are larger, more produced and recurved, so that the caudal or posteal portion of the shell 

 forms a smaller relative proportion of the whole ; the costae have their upper or posteal 

 portions less attenuated, they are also distinctly crenulated, and the crenulations upon 

 the costse generally are more unequal, larger, and more irregular, thus forming a 

 peculiarly roughenened surface. The posteal portion of the narrow area in T. caudata 

 forms a series of large, transverse, very irregular and prominent costellse ; in T. scabricola 

 the area has the rugae regular and inconspicuous ; the wide interstitial spaces in T. caudata 

 have each a plain median small costa, a feature which is altogether absent in T. 

 scabricola. There is required, therefore, only a fairly represented series of each form to 

 demonstrate their distinctness as species. The escutcheon is peculiarly large and deeply 

 excavated, exceeding those features in T. caudata ; its transverse costellse are similar in 

 character, but are more rugose, their upper extremities forming a peculiarly jagged 

 irregular outline upon the upper border. Measurements of a specimen not of the largest 

 dimensions: — Length upon the angle of the valve 31 lines; across the area and escutcheon 

 6 lines; across the pallial surface 18 lines; thickness anteally through a single valve 

 11^ lines. 



Two foreign allied forms of the genus require comparison and separation. The 

 large species figured for T. aliformis by Goldfuss (' Petref.,' tab. 137, figs. 6, 6 a) has 

 the posteal portion of the shell wider and less attenuated ; the upper or attenuated 

 portions of the costse are without the flexure seen in T. caudata and T. scrabricola ; th# 

 anteal portion of the shell is more produced and its costse are concentric ; they are, 

 therefore, without the abrupt obliquity of T. scabricola. 



T. jjlicato-costata, Nyst and Galeotti (' Bull, de I'Academie Roy. Bruxelles,' tome vii, 

 2 partie, p. 221, fig. 1, 1840), from limestone at Tehuacan, in Mexico, was referred by 

 them to Jurassic strata, an error which was corrected by D'Orbigny (' Prodrome de 

 Paleont.,' vol. ii, p. 240, No. 605), who placed the species in his Etage 22 Senonien, 

 or highest stage characterised by Cretaceous Trigonia. The figure, which appears to be 

 very well drawn, has the anterior side much more produced and the posteal portion less 

 attenuated than in T. scabricola ; the costse, which are similarly crenulated, are nearly 

 concentric anteally, are slightly curved in the same direction as the smaller or posteal 

 rows ; their direction, therefore, differs materially from the corresponding parts in the 

 British species. The escutcheon and area, on tlie other hand, appear to correspond very 

 nearly with T. scabricola. 



StratiffrapJncal position and Localities. — T. scabricola accompanies T. aliformis in 

 the beds of the Blackdown and Haldon Greensand. Certain internal moulds brought to 

 my notice by Mr. Cunnington from the Upper Greensands of "Wiltshire probably belong 



