from the Inferior Oolite of the Cotteswolds. 231 



marginal carina ; the entire form is smaller and unlike T. costata ; 

 the area is alike in both the valves. 



It is somewhat rare ; all the examples have been obtained in 

 the upper division of the Inferior Oolite. 



Professor Buckman has obtained it near Cheltenham ; my own 

 specimens are from the Gryphite grit of Rodborough Hill near 

 Stroud. 



Trigonia hemispharica, Lycett^ n. s. PI. XI. fig. 2. 



Shell small, its length not exceeding 3 lines, very much arched, 

 so that the diameter through both the valves slightly exceeds the 

 length ; the umbones are scarcely recurved, acute, contiguous ; 

 the area is large, flattened, forming a considerable angle with 

 the other portion of the shell ; it has numerous fine longitudinal 

 plications faintly traced, there is no median carina, and the inner 

 carina is very small j the marginal cai'ina is acute, elevated and 

 finely serrated ; the other surface has numerous closely aiTanged 

 longitudinal costse, which are united to the marginal carina. 



The large number of costae and the characters of the area in- 

 duce me to regard this as the adult condition of the species, 

 notwithstanding the small dimensions. 



A single specimen is my authority ; it is from the bed of hard 

 pale calcareous mudstone, a local deposit which in the Nailsworth 

 valley replaces the bed of Oohte marl and abounds with Nerinaa. 



Trigonia costatula, Lycett. PI. XI. fig. 5. 

 Trigonia costatula, Lycett in Ann. & Mag. Nat. Hist. 1850, p. 421. 



Shell subtrigonal, convex; umbones mesial, not prominent nor 

 recurved, anterior side produced and rounded, posterior side 

 truncated; area flattened, finely striated transversely, divided 

 longitudinally into two equal portions by a groove and bounded 

 by two low carinse; the marginal carina is imperfectly seiTated, 

 it is of moderate size and but httle curved ; the inner carina is 

 nearly smooth ; the space between the inner carinse is smooth and 

 very narrow or lanceolate; the costae are numerous (about 21), 

 moderately prominent and closely arranged ; they are but little 

 curved, are separated from the marginal carina by a plain sur- 

 face, their direction being nearly horizontal or confonnable to 

 the inferior border. In the ultimate stage of growth the costae 

 posterioi'ly are broken more or less into several portions, which, 

 however, continue to follow the general direction of the costae. 

 In the immature form the costae are not separated from the 

 marginal carina, and the area is traversed transversely by an equal 

 number of prominent plications ; but these gi-adually vanish, and 

 the costae become disunited from. the carina, which then Ijecomcs 



