﻿SCABR^. 137 



Shell subovate ; moderately convex mesially ; rather depressed near the circum- 

 ference at the borders of the valves ; umbones small, obtuse, little produced, and slightly 

 recurved ; anterior border produced, curved elliptically with the lower ])order ; hinge- 

 border slightly convex, moderately lengthened, its extremity forming an obtuse angle 

 with the siphonal border, which is somewhat curved, its length being equal to two thirds of 

 the hinge-border ; its lower extremity is curved with the palhal border. The oblique 

 divisional angle of the valve is well defined ; from it diverge on each side a series of 

 narrow, elevated, closely arranged, spinose costae, which pass each other with a slightly 

 oblique curvature across the opposite portions of the valve ; those which cross the area 

 and escutcheon are the smaller ; and, in conformity with the general figure of the valve, are 

 also the more closely arranged ; the pallial costse increase in size downwards towards the 

 lower border, the more anteal ones curving moderately forwards ; the spines upon the 

 high-ridged pallial costa) are obtuse and erect ; each has its perpendicular plication upon 

 the sides of the narrow ridge; the spines upon the area and escutcheon are much smaller 

 and have little prominence. The surface of the escutcheon is narrow and slightly 

 depressed ; a small elevation separates it from the surface of the area, which is somewhat 

 concave, and destitute of any mesial furrow. 



Dimensions of a lart/e specimen. Length, measured upon the divisional angle of the 

 valve, 25 Hues ; across the valve at right angles to the divisional angle, 21 lines ; con- 

 vexity of a single valve 5 lines. 



Affinities and differences. I am unable to identify with Parkinson's species the 

 T. spinosa of D'Orbigny (' Pal. Fran., Terr. Cret.,' vol. iii, p. 154, pi. 297). The latter 

 has the general convexity much greater ; the umbones are more produced, the area is 

 more concave, and is pointed at its lower extremity ; it has also a mesial furrow ; the 

 escutcheon is wider, shorter, and more concave, so that, when the valve is placed upon its 

 side and viewed from above, the escutcheon is but partially seen. The pallial costse have 

 much greater curvature ; they have delicate crenulations, but are without prominent 

 obtuse spines ; it does not appear to agree strictly with any British species. 



The same author in his ' Prodrome de Paleontologie,' vol. ii, p. 161, separated from 

 T. spinosa a supposed allied form under tlie name of T. PyrrUa, characterised by the few 

 following words : " espece voisine du T. spinosa, mais avec des cotes et des tubercles bien 

 plus gros." 



Having due regard to the erroneous figure and description of T. spinosa above 

 mentioned, taken in connexion with the few words assigned to T. PyrrJia, an impression 

 is conveyed that the latter form is the real representative of the species indicated by 

 Parkinson and Sowerby. 



Agassiz figured for T. spinosa (' Trigonies,' pi. 7, figs. 4—6) a little mould of an 

 external cast from Upper Greensand of the Underclifl", Isle of Wight ; the reader will 

 find it figured and described in the present Monograph under the name of Tric/onia 

 Archiaciana, p. 140, Plate XXV, fig. 10 (mould). 



