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BRITISH FOSSIL TRIGONLE. 



series of regular, transverse, strongly defined costellae, which also pass across the narrow- 

 area to the divisional angle of the valve in specimens of immature growth, but in fully 

 developed examples the posteal portion of the area is destitute of costellae. The area and 

 escutcheon together form a wider surface than in T. aliformis ; the escutcheon is less 

 distinctly bounded, its costellae are smaller and somewhat more numerous than the costae 

 upon the other portion of the valve ; the separation of the two portions of the surface at 

 the angle of the valve is clearly defined, and for the most part forms a narrow divisional 

 ridge, from the sides of which both costae and costellae diverge with great regularity. 

 These features, and more especially the transverse ridges or costellae upon the escutcheon 

 throughout its length, afford a strong contrast to the delicate or evanescent surface- 

 ornaments upon that portion of T. aliformis. 



It may be mentioned, as affording an explanation of the supposed identity with 

 T. aliformis, that well-preserved specimens of T. Vectiana in the Perna-bed at Atherfield 

 are rare, and that the moulds of external casts at Black Gang Chine would readily be 

 mistaken for T. aliformis without a careful comparison of the escutcheon in each 

 instance. 



A small example of the Scabra, figured by Agassiz (' Trigonies,' p. 33, pi. x, 

 fig. 11) under the name of T. plicata, appears to be nearly allied to, and may be identical 

 with, T. Vectiana ; but, as it is founded upon a single specimen only, and deprived of the 

 test, it is not possible to make any satisfactory comparison with it ; apparently the 

 crenulated costae are somewhat more numerous than in the British species. Agassiz 

 believed that his specimen was obtained in the Portland formation (zone of the Calcaire 

 a Pteroceres) in the environs of Besancon. My whole experience is opposed to this 

 conclusion, neither does there appear any reason to doubt that it was obtained from the 

 Cretaceous rocks. 



The Indian T. Forbesii, described and figured with T. aliformis, has the general 

 figure more ventricose, so that the area and escutcheon have greater breadth, the posteal 

 portion of the shell is scarcely attenuated, the umbones are much less elevated, the 

 costae are without curvature, fringed with a few nodes, and are without the delicate, 

 closely set crenulations, both upon their summits and sides, which characterise T. 

 Vectiana. 



The T. aliformis of Pictet and Renevier from the " Terrain aptien " of the South of 

 Prance, above cited, represents a finely preserved example of T. Vectiana, much 

 exceeding the size of specimens occurring in the Perna-bed at Atherfield, the dimensions 

 of which are fairly represented by our figures ; these are of not fully developed growth. 

 The other figure of T. aliformis in the work of Renevier is an undoubted Blackdown 

 specimen of that species. 



Stratiorap/iical positions and Localities. — T. Vectiana occurs somewhat rarely in the 

 lowest, or Perna, bed stratum of the Neocomian formation at Atherfield, Is'e of Wight. 

 At Black Gang Chine, in the same vicinity, its moulds and casts occur abundantly 



