﻿SCABRtE. 117 



the border of the area as narrow, rounded, crennlated ridges, and diverge in every 

 direction ; about seven costse nearest to the apex are concentric or are curved obliquely ; 

 the next succeeding seven, or more, enlarge or become inflated at their middle portions, 

 and pass obliquely downvrards to the pallial border ; their crenulations are faintly traced 

 and irregular, formiug obtuse, transverse nodes upon the costse, which become attenuated 

 as they approach the pallial border. The more numerous and smaller costae occupy the 

 more flattened or posteal portion of the shell ; they are small, narrow, rounded, very 

 closely arranged, minutely crenulated, and nearly perpendicular; their extremities render 

 the lower border dentated. The middle or inflated costse form a slight undulation 

 approaching to a falciform flexure ; the interstitial spaces are plain. The narrow posteal 

 portion of the shell, with its closely placed perpendicular costae and depressed surface, 

 contrasts strongly with the inflated anteal surface with its more widely separated costae 

 enlarged mesially and attenuated at their extremities. Usually specimens of adult growth 

 have upon the anteal face of the valve and adjacent to the borders numerous sniaU, rather 

 obscure, horizontal ridges, or supplementary costellse, which occupy the intercostal spaces 

 of the first-formed six or seven cost«e, a feature which is only visible in well-preserved 

 specimens. The change from the inflated anteal surface to the depressed and flattened 

 posteal portion is abrupt and strongly characterises the species. 



Tlie inner borders of the valves are dentated by the extremities of the pallial costse ; 

 the narrow flattened surface forming the inner border of the escutcheon has a numerous 

 series of small transverse pits ; the narrow produced siphonal border is gaping, and 

 contracted mesially by a projecting longitudinal internal rib in each valve placed beneath 

 the mesial furrow of the area and serving to separate the incurrent and excurrent 

 orifices ; the other borders of the valves are close fitting. 



Dimensions of an unusually fine specimen in the collection of Mr. Vicary and intended 

 to be figured upon a future plate: — Length of the angle of the valve 27 lines; length 

 fi-om the pedal border to the siphonal border 26 lines; height 21 lines; diameter 

 anteally through the united valves 14 lines; breadth across the area and escutcheon 9 

 lines ; length of the siphonal border 5 hues. 



T. aliformis also occurs as a distinct variety and in some abundance in the highest 

 Greensands of Wiltshire at Warminster, and of the Isle of Wight at Ventnor ; the fossils 

 are invariably deprived of their tests, and are usually flattened from vertical pressure. 

 Two uncompressed specimens are represented (Plate XXV, figs. 5, 6) ; the surface 

 ornaments are sufficiently distinct excepting upon the area and escutcheon, where they 

 do not appear to diff'er materially from the corresponding portions of the typical form. 

 Compared with the latter, the figure is more produced and attenuated posteally and less 

 inflated anteally ; the change from the smafl posteal perpendicular costse to the 

 larger, curved, middle series is much less abrupt, and, as the umbones are less recurved, 

 they are more erect, and are nearer to the anterior side, which is shorter; the 

 escutcheon is also more lengthened. Possessing these difi'erences, which are well 



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