﻿144 



BRITISH FOSSIL TRIGONLE. 



extremities ; the concentric curvature of the costge is less considerable than in adult 

 forms. 



The specimens examined are moderately numerous, consisting of valves either united 

 or separated ; their tests are converted into carbonate of calcium, and measure from six 

 to twelve lines across ; they vary somewhat in the closeness with which the costae are 

 arranged and in the size of the costellae upon the area and escutcheon ; the costellae also 

 vary in their number ; thus, some examples have the extremities of the costae and 

 costellae meeting at the angle of the valve with corresponding regularity over nearly the 

 entire length of the area ; in other specimens there is no near corresponding order, the 

 costellae are then more numerous and smaller. 



Affinities and Differences. In general aspect this small species has a considerable 

 resemblance to T. Archiaciana : but the outline is more nearly circular; the umbones are 

 less produced or more obtuse ; the general convexity is greater ; the costae are more 

 numerous and more closely arranged ; they have less prominence and greater curvature 

 anteally ; their intercostal spaces are therefore narrower, and their plications have 

 much less prominence near the pallial border. The costellae on the area and 

 escutcheon are also smaller and more numerous; for the most part they touch the 

 extremities of the costae at the angle of the valve, which does not form a distinct narrow 

 dividing ridge as in T. Archiaciana ; the escutcheon is shorter, wider, or more 

 horizontal ; the siphonal border is more lengthened. 



Compared with T. spinosa, the valves are of smaller size and have greater convexity ; 

 the umbones are larger and more prominent ; the area is smaller, its slope is compara- 

 tively steep, its surface forming a more considerable angle with the other portion of the 

 valve ; the differing features presented by the costae are also very conspicuous ; the 

 smallness and close arrangement of the rows, their little prominence, their rounded upper 

 borders, and their considerable or concentric curvature, so distinct from the high-ridged 

 nearly straight costae and obtuse spines of T. spinosa. 



Dimensions. A large specimen has the length, measured upon the carina, of 12 lines ; 

 across the valve at right angles to the carina, 10 lines, of which the area occupies 

 3^ lines ; length of the escutcheon 7 lines ; length of the siphonal border 5^ lines ; 

 diameter through the united valves 6 lines. 



The test appears to have considerable thickness ; the hinge-processes are usually 

 large for so small a species. 



Stratigraphical position and Locality. All of the specimens known have 

 occurred in a bed of phosphatic nodules in the Fen - district of Cambridge- 

 shire. The position and organic contents of this bed have been investigated by 

 palaeontologists connected with the University of Cambridge, and have been referred by 

 them to the Lower Greensand ; the results of their observations are embodied in several 

 descriptive notices in the ' Geological Magazine' from 1866 to 1868 inclusive, consisting 

 of communications by Mr. J. F. Walker, Mr. H. Seeley, and Mr. H. Keeping; to the 



