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



much larger tubercles ; they are about half as numerous as in 

 T. decorata ; lastly, the few primal costse are smooth, in the other 

 species they are tuberculated. 



A specimen in the cabinet of the author, and a second in that 

 of Dr. Wright, are the only examples with which I am acquainted ; 

 they are nearly of equal dimensions, and agree in all their cha- 

 racteristic features. 



Length upon the marginal carina If inch, opposite measure- 

 ment \^ inch. 



The building-stone of the Inferior Oolite which forms the 

 upper portion of its middle division is the seat of this species, 

 which has been obtained in the vicinity of Nailsworth. 



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 carinse are prominent and dentated, the latter 

 separated from the longitudinal costse by a depression ; lanceo- 

 late space between the inner carinse reticulated; longitudinal 

 costse slightly undulated, with a graceful double curvature re- 

 sembling an elongated letter/. 



Var. 3. 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 costse equal in nimiber 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 30 lines, but few specimens are 

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

 the intercarinal plications are prominent ; in the Inferior Oolite 

 it occurs in the freestone beds. 



