82 BRITISH FOSSIL TRIGONI^. 



made for these adverse circumstances, it will readily be understood that the specific 

 identity of the two forms, apparently dissimilar, was not at first discovered, and that they 

 were believed to represent different species ; the cream-coloured rock, which also abounds 

 with the genus Nerincea in the vicinity of Stroud and Nailsworth, is little used for 

 economical purposes, and the Trigonia therefore has very rarely been obtained. 



Trigonia cosiatula appears to occupy in its sectional characters a position between the 

 costat(B and the undulatcB, or to connect these groups, but as the area and escutcheon 

 agree with those parts of the imdulata, and are altogether distinct from the costata, I 

 have preferred to place it with the former section, notwithstanding the presence of a series 

 of plain horizontal costse upon the other portion of the shell. 



Specimens of this rare species are in the Museum of the Royal School of Mines ; 

 in the Woodwardian Museum, Cambridge; also in the collections of Dr. Wright, 

 Cheltenham ; Mr. Witchell, Stroud ; Rev. P. B. Brodie, Rowington, near Warwick ; and 

 in my own cabinet ; all from the middle portion of the Inferior Oolite in the vicinity of 

 Cheltenham and of Stroud. 



Trigonia Joassi, Lycett, sp. nov. PI. XX, figs. 2, 3, 4. 



Shell ovately oblong, convex ; umbones placed within the anterior third of the valves, 

 moderately elevated; anterior side short, curved elliptically with the lower border; 

 posterior side much produced and somewhat depressed, pointed at the extremity of the 

 marginal carina. Area of considerable breadth, flattened, rugosely plicated transversely. 

 Escutcheon lengthened, narrow and flattened ; marginal carina slightly elevated, trans- 

 versely knotted. The other portion of the surface has the rows of costae very 

 numerous, each row having a double undulation resembling in figure the radii upon the 

 Ammonites of the group of Falciferi, but with less regularity. The nodes in the rows 

 are usually small, with much inequality in size and variability in figure, but for the most 

 part they are either rounded or ovate ; anteally they become much attenuated or cord- 

 like ; they are curved upwards obliquely from the anterior border, and at the end of 

 about two fifths of their course form a sudden flexure directed obliquely downwards and 

 become more distinctly nodulous. The nodes are irregular in the rows, and about the 

 middle of the valve some of the anteal rows terminate ; the remainder of the rows form 

 another undulation, curving upwards to the marginal carina at a considerable angle, 

 becoming more ridge-like and attenuated at their extremities. The nodes upon the 

 middle portion of the valve are commonly confused, unequal in size, and irregular in 

 figure ; but this does not appear to be an invariable feature, as a specimen in the collec- 

 tion of Mr. Grant, of Lossimouth, has the rows of nodes regularly falciform and nearly 

 equal. This specimen, although imperfect, shows that the rows near to the umbones 



I 



