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BRITISH FOSSIL TRIGONLE. 



distinct anteal truncation ; the marginal carina of the right valve is much larger than 

 that of the other. Numerous examples have been brought from Western Australia, but 

 the locality and geological position have not been exactly ascertained. 



Trigonia denticulata, Ag. Plate XXIX, figs. 1, 2, 3, 4. 



Trigonia costata, Zieten. Petref. Wiirtemb., tab. cxxxvii, fig. 3 a, b, 1838. 



— denticulata, Agassiz. Trigonies, p. 38, tab. xi, figs. I — 3,1840. 



— scuticulata, d'Orbigny. Prodrome de Paleont., vol. i, p. 278, No. 314, 1850. 



— denticulata, Sharp. Oolites of Northamptonshire, Quart. Journ. Geol. 



Soc., vol. xxvi, p. 388, 1870. 



— — Phillips. Geology of Yorkshire, vol. 4 i, p. 250,3rd ed., 1875. 



— — Judd. Mem. Geol. Surv., Rutland, &c, pp. 153, 281, 1875. 



Shell smaller than T. costata, more ovately trigonal, and less convex; umbones 

 prominent, pointed, much incurved and more or less recurved ; anterior border produced, 

 curved elliptically with the lower-border ; hinge-border straight, sloping obliquely ; its 

 posteal extremity forms an obtuse angle with the siphonal border of the area. The area 

 is wide and flattened, the plane of its surface forming a considerable angle with the 

 surface of the other portion of the valve ; it is bounded by two well-marked denticulated 

 ■carinae, having also in the left valve occasionally a small median carina or a costella 

 somewhat larger than the others and separating the area into two portions, the superior 

 •one of which is depressed and concave ; the intercarinal spaces have numerous small 

 denticulated costellae which vary much in their prominence in different specimens ; in the 

 right valve the costellae are fewer and more irregular and unequal ; there is a median 

 .groove but no distinct median carina ; the marginal carina is prominent, rounded, and 

 denticulated even to the apex; the escutcheon is lengthened, moderately wide, and slightly 

 depressed ; its superior border is somewhat elevated, its ornamentation consists of very 

 small diverging delicately serrated plications. The other portion of the valve has the 

 costae differing much in numbers, narrow, usually numerous, horizontal, curved upwards 

 to form a slight undulation anteally, so that all the costae terminate at the anterior 

 border. Specimens differ in the length, measured in the direction of the costa3, and also 

 in the general convexity. 



Examples from Cloughton have not uncommonly the epidermal tegument preserved 

 over a considerable portion of the surface ; the lines of granules are large and closely 

 arranged ; the matrix of soft shale appears to be the cause of this favorable condition of 

 preservation. The lines of growth are peculiarly delicate and densely arranged. 



This is an elegant and moderate-sized species of the Costata ; specimens differing 



