258 Mr. J. Lycett on some new species of Trigouia 



Trigonia costata. 



Trigonia costata, Lam., Sow., Zeiten, Deshayes, &c. 

 Typical form Costata. 



Anterior border truncated; umbones prominent, recurved; 

 area slightly concave, with denticulated oblique plications, which 

 differ in the two valves, the area of the right valve having two, 

 three, or four large plications upon its anterior half, and no di- 

 stinct median carina ; the area of the left valve has a distinct 

 median carina, and four or five plications upon each side of it ; the 

 inner and marginal carinas are prominent and dentated, the latter 

 separated from the longitudinal costse by a depression ; lanceo- 

 late space between the inner carinas reticulated; longitudinal 

 costae slightly undulated, with a graceful double curvature re- 

 sembling an elongated letter/. 



Var. 2. multicosta. 



This variety is much smaller than the typical shell ; it is some- 

 what more depressed, its anterior border scarcely truncated, the 

 area more finely reticulated, the costas equal in number to the 

 typical form, but more delicate and closely arranged ; multicosta 

 only attains about half the linear dimensions of the typical form ; 

 it has occurred only in the bed called Gryphite grit. 



Var. 3. pulla. T.pullus, Sow. 



This small variety occurs both in the Inferior and Great Oolite 

 of the Cotteswolds ; the larger specimens have a length upon the 

 marginal carina not exceeding 20 lines, but few specimens are 

 so large. The anterior border is not truncated, the carinas and 

 the intercarinal plications are prominent ; in the Inferior Oolite 

 it occurs in the freestone beds. 



Var. 4. sculpta. 



In dimensions this well-marked variety equals the typical form ; 

 in ctrtain localities it occurs in the Gryphite grit in immense 

 abundance ; it is distinguished from the typical form by several 

 conspicuous characters ; the figure is less trigonal, the anterior 

 border being destitute of any truncation ; the umbones have less 

 prominence and are less recurved, the area is larger, flatter, it is 

 less concave, and occupies a much larger proportion of the sur- 

 face of the shell ; the marginal and inner carinas are larger, less 

 curved, and in common with the intercarinal plications, they are 

 much more strongly dentated ; owing to this prominence of the 

 plications the median carina is much less conspicuous, the pos- 



