28 BRITISH FOSSIL TRIGONLE. 



condition ; the largest has a length of six inches upon the marginal carina ; the opposite 

 measurement is four and a half inches ; but, judging from other imperfect specimens, these 

 are not the largest dimensions attained by the species. 



M. Seebach states that T. triquetra occurs in the Coralline Oolite of Malton and 

 Pickering, but of this I know of no example ; in Hanover he gives as its locality the 

 Hersum beds of the Tonnjesberg, in true Coral Rag. Specimens are in the Museum of 

 Practical Geology, in the Museum of the Philosophical Society, Scarborough, and in 

 the cabinet of the author. 



For the larger of the specimens figured (Plate VI, fig. 2) I am indebted to the gene- 

 rosity of Mr. Hawkridge, of Scarborough. 



Trigonia Rupellensis, D' Orb. Plate VIII, fig. 4. 



Trigonia Rupellensis, D'Orbigny. Prodrome dePaluont., 1850, tome ii, p. 17, No. 261. 

 — clavellata, var. Leckenby. Quart. Jour. Geol. Soc, 1859, vol. xv, p. 8. 



Shell ovately trigonal, moderately convex ; umbones elevated, pointed, and slightly 

 recurved, placed within the anterior third of the valves ; anterior side short, both it and 

 the inferior border elliptically curved ; posterior side moderately produced, its border 

 somewhat concave. Escutcheon lengthened, narrow, depressed, and flattened ; its length is 

 equal to two thirds of that of the marginal carina ; the area is narrow and flattened, 

 delicately transversely plicated, with three very small minutely knotted carina?, which 

 become evanescent posteally ; the posteal extremity of the area forms an obtuse angle, 

 both with the escutcheon and with the inferior border ; the apical portion of the area has a 

 few transverse costellse. The costated portion of the shell has about thirteen rows of 

 tuberculated costse, of which the four or five rows first formed are simply sub-concentric 

 and ridge-like or sub-tuberculated ; all the remaining costse have distinct rounded tuber- 

 cles, but their direction is very irregular ; posteally they are curved upwards to the 

 carina at a considerable angle, and the last three or four costse have their superior 

 extremities with the tubercles confluent or forming small depressed varices ; anteally 

 there are two or three short additional or supplementary rows of tubercles, which form a 

 very irregular and confused ornamentation over a large portion of the valve ; nevertheless, 

 the anteal extremities of the rows pass to the border in regular order and attenuated 

 form. 



The plications of growth are strongly defined ; they impress the three last-formed rows of 

 costse. The length of the marginal carina is about one fourth greater than the height ; 

 the diameter through the united valves is equal to half the height. 



Only a few specimens have been procured, and these vary much from each other in 



