66 BRITISH FOSSIL TRIGONI.E. 



Upper Lias : T. navis. Lam., T.pulchella, Ag., T. tuberculata, Ag. {T. spinidosa, Y. and 

 B.), T. similis, Bronn, and T. costellata, Ag. Subsequent researches have shown that 

 the latter five species are associated more or less one with another in a single geological 

 position, and that two or more of them occur together at several localities in Southern 

 Germany. Professor Quenstedt (' Der Jura.') has established T. navis and T. pulchella as 

 species of the lower portion of the Inferior Oolite. 



In Britain T. spinulosa pertains both to the Supra-Liassic Sands and to the lower 

 portion of the Inferior Oolite ; and, as the two remaining species are associated in Southern 

 Germany with the three former, it may be inferred that all of them belong to a higher 

 position than T. literata, and that the latter therefore occupies the lowest position of any 

 known species of the Upper Lias. 



In Britain T. literata has occurred only at a single locality ; namely, a little higher than 

 the middle of the Upper Lias Shale at the Peak, Robin Hood's Bay, in scars accessible only 

 at low water ; there are no Ammonites in this stratum, but it immediately overlies a bed 

 with Ammonites crassus, Y. and B. ; it is lower than the beds worked for alum upon the 

 same coast. Specimens occur of every stage of growth, with the valves both united and 

 separated ; but, as the greater number have the characters of the surface ill-preserved, good 

 specimens are somewhat rare. 



Trigonia V-costata, Li/c. Plate XIII, fig. 5 ; Plate XV, figs. 1, 2, 3, 4. 



Trigonia angulata, Phil. Geol. York., 1829, vol. i, p. 156 (not Sow.). 



— — Williamson. On Distribution of Fossils, Yorkshire Coast, Trans. 



Geol. Soc, 1836, 2 ser., iii, p. 229. 



— v-cosTATA, Lycett. Ann. and Mag. Nat. Hist., 1850, p. 422. 



— — Morris. Catalogue, 1854, p. 228. 



— — Lycett. Cotteswold Hills Handbook, 1857, pi. vi, fig. 5. 



Shell ovately trigonial, moderately convex ; umbones nearly mesial, produced, obtuse, 

 and usually somewhat recurved ; anterior side produced, its border curved elliptically 

 with the lower border; hinge-border slightly concave, sloping obliquely, its extremity 

 forming an obtuse angle with the extremity of the area. Area narrow, concave beneath 

 the apices, but flattened posteally ; it is traversed transversely by delicate plications, 

 which near to the apices form a few regular, smooth costellse ; it has a mesial longitudinal 

 furrow, and in the young condition three closely tuberculated carinas, which become 

 evanescent posteally with advance of growth. The escutcheon is much depressed compared 

 with the inner carina; it is lengthened, of moderate breadth, and perflectly flat. The 

 other portion of the surface has the rows of costse numerous (twenty to twenty-four) and 

 narrow ; they are but little raised, and are rather inconstant in their characters ; sometimes 



