56 BRITISH FOSSIL TRIGONIiE. 



with little prorainencej erect, or in other examples slightly recurved; anterior side 

 moderately produced, its border curved elhptically with the lower border ; hinge-border 

 lengthened with some concavity, or in other instances nearly straight, sloping obliquely 

 downwards ; its length is nearly equal to twice that of the posteal border of the area, 

 with which it forms a considerable angle. Escutcheon lengthened, much excavated, but 

 having its superior border raised. Area of moderate breadth, concave immediately beneath 

 the apex, but expanded and flattened posteally ; it has regular, transverse, prominent plica- 

 tions, which become costellae near to the apex ; there is a well-marked mesial furrow ; the 

 bounding carinse are small, but elevated and distinct ; they have small, closely arranged, 

 ovate tubercles or varices throughout their entire length ; there is no median carina. 

 The other portion of the valve has the rows of costse rather numerous (about sixteen) 

 anteally ; they are plain, narrow, depressed, and horizontal, or are directed slightly 

 downwards to about the middle of the valve, where they enlarge, form two or three 

 nodose varices, and, curving gracefully upwards, become again suddenly attenuated, and 

 meet the marginal carina at a considerable angle ; in some instances, as in fig. 9, the 

 costse become broken mesially, and form an imperfect angle with their posteal portions. 

 In adult specimens two or three of the last-formed anteal costse coincide in their direction 

 with the lines of growth ; they therefore take the direction of the lower border, which 

 is without any undulation, as in T. cmgulata ; the anteal costse are always somewhat more 

 numerous than the others. 



Affinities. The nearest ally is T. an(julata, compared with which it is more depressed 

 both anteally and mesially ; its posteal portion is more expanded, and its lower border is 

 destitute of the posteal undulation of that species; its costae are also more numerous 

 anteally, and do not form a distinct undulation or double curvature upon the middle of 

 the valve, so that their general direction accords more nearly with the lines of growth ; 

 their posteal portions are also larger and broader. 



T. Painei, another species of the Great Oolite, has also considerable affinities with our 

 species ; the latter has the form more lengthened posteally, the umbones are less 

 produced, and are more anteal ; the costse are more numerous ; its area more especially 

 differs in having delicately tuberculated carinse and a rugose plicated surface. 



T. paucicosta, of the Kelloway Rock, has greater general convexity, and its area, with 

 the peculiarity of its few, large, widely separated tubercles upon its carinse, will readily be 

 distinguished. 



The young shells of T. flecta offer little that is distinctive from specimens of similar 

 size pertaining to T. angulata and T. Painei ; the transverse costellse upon the area are, 

 however, smaller than in the last-named species ; compared with T. paucicosta their 

 general ornamentation is much less conspicuous, more especially upon their carinse. 



The largest of our specimens has the length of 23 lines, height 19^ hues, diameter 

 through the united valves 10 lines. 



Stratif/rapMcal position and Localities. T. flecta appears to be a somewhat rare species; 



