64 BRITISH POSSIL TRIGONI^. 



the Woodwardian Museum, Cambridge ; also in the author's cabinet. It is now for the 

 first time figured. 



Trtgonia literata, Young and Bird. PI. XIV, figs. 1, 1«, 2, 3, 4. 



Trigokia literata, Young and Bird. Geological Survey of the Yorkshire Coast, 2nd 



ed., 1828, p. 225, pi. viii, fig. 23. 

 _ _ Phillips. Geol. of Yorkshire, 1829—1 835, vol. i, pi. xiv, fig. 1 1. 



— — Williamson. Distribution of Fossil Remains on the Yorkshire 



Coast, Tr. Geol. Soc, 2nd series, 1836, vol. v, 

 p. 243. 



— — Pusch. Polens Palseont., 1837, p. 60. 



— iiTTERATA, Agassiz. Trigonies, 1840, pp. 8 and 50. 



— — Brown. Foss. Conchol. of Great Britain, 1845. 



— LYRATA. D'Orhigny. Prodr. de Paleont., 1850, vol. i, p. 218. 



— LiTTERATA, Morris. Catal. 1854, p. 229. 



— IITTERATA, /S'mpsow. Fossils of Lias, 1855, p. 116. 



— UTTERATA, Quenstedt. Der Jura., 1856, p. 442. 



— — Oppel. Juraformation, 1856, p. 260. 



— LiTTEEATA, Tate. Geol. Mag., 1872, vol. ix, p. 306. 



Shell sub-ovate or ovately oblong, convex; umbones large, moderately elevated, 

 obtuse, nearly erect, placed within the anterior third of the valves ; anterior side 

 moderately produced, its border curved elliptically with the lower border ; superior 

 border lengthened, nearly straight, sloping obliquely downwards, and forming posteally 

 nearly a right angle with the posterior border of the area. Escutcheon wide and some- 

 what concave, its superior border is moderately raised. Area narrow, slightly convex, 

 with a well-defined mesial furrow. Young examples have two distinct bounding carinse ; 

 the inner carina is characterised by transverse narrow irregular varices ; the marginal 

 carina has irregular widely separated tubercles ; in examples of more advanced growth 

 the carinse have small varices which are united to the transverse plications upon the area ; 

 much variability exists in the prominence of the plications, but usually specimens of full 

 development have the posteal portions of their areas characterised only by delicate lines of 

 growth, and are altogether without ornamentation. The other portion of the surface has 

 two distinct series of tuberculated costse, this distinctness commences at the apices even 

 of the youngest specimens; the anteal series has the rows very numerous, small, and 

 extremely irregular ; in young specimens they approach the anterior borders horizontally, 

 as smooth attenuated lines ; with the curvature of the valve they become sub-tuberculated, 

 and are usually deflected slightly downwards, but their direction is scarcely alike in any 

 two specimens ; they are always small and unsymmetrical, one row with another, unequal, 

 and either prominent or obscure, sometimes partially united to the extremities of the 

 larger posteal series, or altogether separated from them and excentric. Well-preserved 



