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BRITISH FOSSIL TRIGONE. 



Altogether the characters of the Costata are more prominently developed in the 

 typical form of T. elongata than in any other British example of the section ; the elevated 

 cushion-like escutcheon and the considerable concavity formed by the upper division of 

 the area separate it both from its varieties and from other allied forms. 



Position and Localities. It is not an uncommon fossil in the Oxford Clay of the 

 southern counties of England ; numerous and fine examples have long been obtained in 

 the Backwater to the rearward of the town of Weymouth. The figures 1 and 2 of 

 pi. ccccxxxi in Sowerby's ' Mineral Conchology ' are good representations of the right and 

 left valves from that locality ; apparently figure 3, which is a French specimen, should be 

 united to T. cardissa, Ag. 



Variety Angustata. A very narrow form, lengthened perpendicularly, and having 

 considerable convexity near the umbones, is depicted in Plate XXX, figs. 1, 1 a, 2. The 

 costoe are numerous, more closely arranged than in the typical form, short, and nearly 

 horizontal, excepting upon the anteal face of the valve, where they have a slight horizontal 

 undulation. The marginal carina is comparatively inconspicuous, with small, numerous, 

 transverse indentations. The surface of the area is similar to that of the typical form, 

 excepting that it is not concave. The escutcheon is lengthened, sloping obliquely 

 downwards ; it has less breadth than in the typical form ; its upper border is convex. 

 Our figures represent specimens of the largest dimensions. 



Position and Localities. The variety Angustata appears to be limited to the Corn- 

 brash of the north of England. My few specimens are from the vicinity of Scarborough, 

 where it has occurred only rarely ; the narrow form, abrupt truncation of the lengthened 

 anterior border, and short, horizontal costa? will usually separate it from another larger 

 and more common variety in the same bed, or Macrocephalus-zone of Quenstedt and 

 Oppel. 



Variety Lata. This, the largest of the elongata group, is almost limited to the 

 Cornbrash, an occasional badly preserved specimen having been obtained in the lowest 

 bed of Kelloway Rock at the same Yorkshire locality. It is moderately abundant, occurring 

 very rarely with the valves united ; for the most part it is ill preserved, especially the 

 surfaces of the area and escutcheon ; our figures, Plate XXX, figs. 4, 5, appear to illustrate 

 it sufficiently. The general convexity is considerable, but scarcely equals that of the 

 typical form. 



There is much variability in the proportions of the general figure ; usually the area 

 has less breadth, and is more elevated, than in the typical form ; its surface forms a 

 smaller angle with that of the other portion of the valve ; its upper or inner division is 

 more flattened, but has some depression ; and the median carina is distinct. The costated 

 portion of the valves varies in breadth and in the distinction of its anteal truncation ; the 

 costse are large, their general direction is oblique, and they have an horizontal undulation 

 upon the anteal surface. The marginal carina in each valve is large, but less prominent 

 than in the typical form. The escutcheon is large and usually flattened ; it slopes obliquely 



