62 MOLLUSCA FROM THE GREAT OOLITE. 



Stonesfield slate, at very many localities, but have not hitherto been discovered in the 

 shelly beds of the Great Oolite. 



Localities. Stonesfield, Eyeford, and generally throughout Oxfordshire and Glouces- 

 tershire, vi^here the Stonesfield slate is present. 



Trigonia Phillipsi. Tab. VI, fig. 1. 



Testa ovato-trigond, convexd, umhonihus suhmedianis obtusis, subrectis, ared parvd 

 planatd ; carind marginali angustd, tuberculis parvis, crebris ornatd, carind interna vari- 

 cibus subdistantibus ; superjicie inter carinis plicis crebris transversis etsulco obliquo mediano 

 instructo ; valvis lateribus costis concentricis regularibiis crebris elevatis, tuberculis parvis 

 dense dispositis. 



Shell ovately trigonal, convex, anterior border produced and rounded ; posterior border 

 truncated, umbones nearly mesial, obtuse, nearly straight and scarcely recurved, marginal 

 carina nearly straight, narrow, and little elevated, ornamented with minute closely 

 arranged tubercles, inner carina with a few prominent rather distantly placed varices ; 

 lanceolate space between the inner carinse wide and smooth ; the surface of the area 

 between the carinse is flattened, traversed transversely by prominent closely arranged 

 plications, and divided in its middle part by an oblique furrow ; the sides of the valves 

 have very numerous elevated narrow concentric regular costae, which are ornamented with 

 small, equal, densely arranged tubercles. 



This elegant shell possesses a considerable general resemblance to Trigonia striata, 

 Sow. ; like that shell the costae are regular, concentric, elevated, and are furnished upon 

 their upper surfaces with small tubercles ; but the figure is essentially different ; T striata 

 has the umbones recurved and pointed, the hinge margin posteriorly much excavated, 

 the marginal and inner carinas have a graceful curvature, and the posterior side of the 

 shell is considerably produced ; none of these features are observable in our species, the 

 hinge margin of which is scarcely concave, the umbones obtuse, not prominent and 

 recurved ; the posterior side is likewise so short that the umbones appear to be nearly 

 mesial ; the costae in our species are nearly twice as numerous, equally elevated, and the 

 minute tubercles upon them are rounded and much more densely arranged, so that a little 

 distance from the eye the tubercles are scarcely distinguishable. This comparison can 

 only be made between the specimens themselves, for it happens that nearly all the figures 

 hitherto published of Trigonia striata are very unsatisfactory, with the exception only of 

 that in the Petrefacten of Goldfuss, which is excellent, and represents the adult condition 

 of that species. 



Trigonia Phillipsi occurs in soft Oolite, in the vicinity of Stamford and Denton, 

 Lincolnshire, and has not been recognised in the Great Oolite of Gloucestershire. 



Dedicated to Prof. John Phillips, whose philosophic researches have greatly contributed 

 to tlie advancement of geological science. 



