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BRITISH FOSSIL TRIGONLE. 



The typical form, which is exemplified by a very good figure in the "Mineral Conchology" 

 of Sovverby, pervades various beds of that formation, and also occurs rarely in the southern 

 Cornbrash ; nevertheless it is not generally an abundant fossil. Compared with the 

 other variety, which I designate lata, it has great convexity ; the form is more lengthened 

 or more pointed, and produced at both of its excremities ; the area has greater breadth ; 

 its surface forms a more considerable angle with the other portion of the shell, so that 

 when a valve is laid horizontally and viewed from above, the area is only partially visible. 

 The variety lata, therefore, has the area somewhat smaller, but more expanded, and the 

 siphonal border is somewhat shorter. 



The following description is intended to apply to examples of the species generally, 

 and not as a minute delineation of any individual specimen. This is rendered necessary 

 by the fact, that examples of the same variety from a single bed and locality are not 

 precisely alike in their general proportions or in the lesser details of their surface orna- 

 ments ; thus, although considerable differences will be found to exist between certain 

 selected specimens, others intermediate render it difficult to arrange such forms into 

 distinct varieties. The separation here adopted will not, therefore, in every instance 

 appear to be well founded. Our figures will, it is trusted, enable the reader to appreciate 

 this subdivision with.greater certainty than would be effected solely by a description. 



Diagnostic characters. Shell subtrigonal, very convex near the divisional angle of 

 the valve and near the apex, rather depressed posteally ; umbo prominent, pointed, 

 incurved, and somewhat recurved ; anterior side little produced, its border truncated, 

 near the base it curves with the somewhat shorter lower border ; the superior border 

 of the escutcheon is slightly convex, its posteal extremity forms an obtuse angle with the 

 siphonal border, which has a sinuated outline. 



The escutcheon is flattened and slightly depressed ; its breadth with the valves united 

 exceeds its length ; it is well circumscribed and is traversed by large obliquely diverging 

 varices or costellae. The area is large and flattened ; it has some convexity near the 

 marginal carina in the right valve ; its superior half is somewhat concave ; its breadth at 

 the siphonal border differs considerably, occasionally it is equal to half the height of the 

 shell ; it is bounded by two plicated carinas, but the marginal carina only has any 

 considerable prominence ; it is large ; its indentations are closely placed and do not 

 deeply impress it, they pass across the costellae of the area uninterruptedly, giving a 

 reticulated surface to that portion of the shell. The inner carina is broad and depressed, 

 or in another variety nodose, its plications crossing the escutcheon as small, waved 

 striations. 



Each portion of the area has from three to five costellae, uncertain in size and 

 number ; the costella which divides the inner from the outer portion of the area is 

 somewhat the larger, forming a median carina ; in the right valve it exists only as one of 

 the four or five outer costellae which are larger than in the other valve. 



