BIVALVIA. 61 



obtuse, slightly recurved ; area narrow, transversely striated with a mesial longitudinal 

 furrow ; carinae small, with densely arranged minute tubercles ; costae serrated ; the first 

 few costae are regular and concentric, the others are directed obliquely downwards from 

 the marginal carina to the lower border, they are nearly straight, some few are dichotomous 

 and slightly waved, the serrations are irregular or unequal, which gives to the costae a 

 knotted aspect. 



This species is not uncommon (more especially the external moulds), in the bed called 

 Triffonia grit, a member of the upper division of the Inferior Oolite ; in the Great Oolite 

 it is very rare. 



Localities. Minchinhampton Common, in the Great Oolite. The Cotswold hills 

 generally, in the Inferior Oolite. 



TRIGONIA IMPRESSA, Sow. Tab. V. fig. 24. 



TKIGONIA IMPRESSA, Sow. Zool. Journal, iii, t. 11. 



Prevott. Ann. Scien. Nat., iv, t. 18, f. 22, 23. 

 Morris. Catal. Brit. Foss., p. 103, 1843. 



Testa ovato trigond subcompressd, antice productd rotundald, postice recta, obliqud ; 

 umbonibus submedianis acutis ; costis per series numerosis Iceviter arcuatis, subundulatis et 

 tuberculatis, costis, anticis, obliquis, anguntis, subrectis dense serratis, posticis curvatis, 

 tuber culls parvis crebris ; area angustd, transversim striatd, striis tenuibus crebris ; carind 

 marginali nodulis parvis regularibus ornatis, carind internd transversim plicatd ; carind 

 media sulco longitvdinali. 



Shell ovately trigonal, rather compressed, anterior side produced and rounded, 

 posterior side straight, oblique, and compressed, umbones nearly mesial acute and very 

 slightly recurved ; costse disposed in a numerous series which are moderately curved, 

 little elevated, somewhat angulated and tuberculated. The anterior portions of the costae 

 are narrow and but little prominent ; they are nearly straight, but are directed obliquely 

 downward, and are more or less distinctly serrated or indented, but do not form distinct 

 tubercles ; posteriorly the costaB are more curved or rather angulated ; they rise upwards 

 to meet the marginal carina at a right angle and are distinctly tuberculated, the tubercles 

 being small and closely arranged. The area is rather narrow and distinctly bounded by 

 two carina? ; the marginal carina is small, it has regular elevated tubercles which are 

 rather distantly arranged, the inner carina is plicated, an oblique furrow or line replaces 

 the median carina, the surface of the area has at first a few prominent transverse plications, 

 but these soon degenerate into striations which are fine and densely arranged ; the 

 lanceolate space between the inner carinse is elongated and smooth. 



The aspect of this little shell is peculiar, and its features are very persistent few 

 exceed an inch in length, and from this size to half an inch is its most frequent dimensions ; 

 they were eminently gregareous, and are numerously scattered over the thin laminae of 



