﻿COSTATJ^. 169 



at right angles to the carina across the valve 45- inches ; convexity of a single valve 

 Igth of an inch. 



A smaller Trigonia occurs in the Coral Rag of Wiltshire, which appears to be a 

 ■variety of T. Meriani ; the general figure has less convexity and is somewhat more 

 pointed at both the extremities ; the other general features are alike in both the forms ; 

 our smallest figure represents this variety. 



T. Meriani has scarcely hitherto been recognised as a British species ; it has occurred 

 only rarely, and in its usually defective condition has been assigned to T. costata ; its 

 aspect when well preserved is sufficiently remarkable both on account of its large 

 dimensions and also for a certain elegance of figure, together with a minuteness and 

 delicacy in the surface ornaments, which might be expected to attract attention, and 

 offers a considerable contrast to the T. costata of the Inferior Oolite. Considering the 

 large dimensions of T. Meriani, its more remarkable characters consist in their general 

 diminutive size and small prominence, such as the smallness and partial irregularity of 

 the costee, the small and nearly plain marginal carina, the inconspicuous median and 

 inner carina, together with the minute and almost evanescent intercarinal costellae. 

 The smallness and irregularity of the costae in so large a species is a feature altogether 

 alone in the Jurassic costata, and is suggestive of a Spanish Neocomian species, 

 T. 2^eninsidaris, Coquand, ' Monogr. de I'Btage Aptien de la Espagne,' pi. xxiii, fig. 3, 

 in which the same feature is more remarkably conspicuous. 



T.monilifera, Ag., from the Coral Rag and Kimmeridge Clay of the southern counties, 

 'has in the general figure some resemblance to T. Meriani and sometimes occurs even of 

 larger dimensions ; it will readily be distinguished by the large, widely separated costae, 

 by the remarkably prominent and strongly indented carinse, and by the great concavity 

 both of the area and escutcheon ; the costellae of the escutcheon also ofiier a minute but 

 not less distinctive feature. 



Hitherto T. Meriani has been very insufficiently figivred ; the drawing intended for 

 this] shell, named T. costata in the work of Young and Bird above cited, is in the 

 usual coarsely executed style of the other figures, and the surface ornaments of the area 

 are altogether erroneous ; the general contour of the valve, the characters of the costae, 

 and of the marginal carina are distinctive and readily recognised. 



A single imperfect and insufficiently characterised specimen was figured by Agassiz ; 

 his description was founded upon better preserved examples subsequently brought to 

 his notice. 



To the present species must also be referred a reduced figure of a costated form in 

 the work of Professor Phillips on the ' Geology of Oxford,' from the Coral Rag of 

 Heddington (PI. xiii, fig. 32) ; printed erroneously T. clavellata. 



Position and Localities. Only a small number of specimens of this large Trigonia 

 have come under my observation ; they have all been obtained in the Coralline Oolite 

 formation at several widely separated localities, as at Weymouth, in Wiltshire, in Oxford- 



