﻿146 



BRITISH FOSSIL TRIGONLE. 



Trigonia Cunningtoni, Lycett, sp. nov. Plate XXIII, fig. 11. 



Shell ovately trigonal or subtrigonal, moderately convex ; its outline forms nearly an 

 equilateral triangle, with the angles rounded ; the umbones are submesial, erect, and 

 obtuse ; the anterior side is short, its border is truncated or nearly straight ; the lower 

 border is gently curved ; the hinge-border is nearly straight, sloping obliquely down- 

 wards, and forming an obtuse angle with the perpendicular posteal extremity of the area. 

 The area, which is of moderate breadth, forms a considerable angle with the other 

 portion of the valve ; it is flattened or slightly convex, having numerous, closely arranged, 

 small, depressed, curved, oblique, scabrous costellse ; there are no traces of carinas ; a 

 simple divisional angle separates the area from the other portion of the surface. 

 The costse are numerous, closely arranged, depressed, and rounded ; they curve obliquely 

 downwards towards the pallial border, and are much attenuated as they approach the 

 oblique divisional or carinal angle ; they are traversed by unequal and irregularly 

 arranged horizontal plications of growth, which also pass over the area ;! each 

 plication as it crosses the costse forms a line of small, rounded, depressed nodes, the 

 direction of which is horizontal or only slightly curved ; fourteen rows are visible upon 

 one imperfect specimen without including others near the umbo where the test has 

 disappeared ; the rows are more closely arranged near the lower border, but their 

 relative distances and the prominence of the rows are extremely variable ; about thirteen 

 nodes occupy each row of longitudinal plications. 



The internal mould is smooth ; it is less trigonal than the test ; the apices are 

 elevated, erect, and widely separated ; the dental impressions are large, and the line that 

 bounds the area is distinct ; the pallial border is deeply crenulated. 



The height and the length are equal ; the diameter through the united valves slightly 

 exceeds half the height. 



Our species is readily distinguished from all other known examples of the Scabra by 

 the peculiar aspect of the closely arranged, depressed, rounded costse, with their rows of 

 small horizontal nodes and transverse plications. 



It appears to have been mistaken for 1. Constants, D'Orb. (' Pal. IV./ pi. 291, 

 figs. 4, 5), and has been thus named in collections ; the latter species is much more 

 lengthened transversely, and more ovate ; its narrow ridge-like costse, and area destitute of 

 costellse, render the ornamentation altogether distinct. 



Stratigraphical position and Locality. The specimen with the test partially preserved 

 was obtained by Mr. Cunnington in the Upper Greensand of Devizes, Wilts ; apparently 

 no other example is known to have occurred at that locality. The British Museum has 

 several fine specimens from Normandy; these are nearly destitute of the horizontal 

 plications and nodes. 



