7() BRITISH FOSSIL TRIGONIiE. 



penclicularly upwards ; the last-formed two or three anteal costse become shghtly waved 

 or irregular, and their direction accords nearly with the lines of growth, which become 

 conspicuous near the lower border ; they also form, near the middle of the valve, an 

 horizontal sulcation or arrest of growth. 



Localities. The materials upon which the foregoing description is founded are some- 

 what scanty, consisting of three British and as many foreign examples. The larger of the 

 specimens figured, with the valves united, is in a very perfect condition, and was obtained 

 by the Rev. P. B. Brodie, in Forest Marble, near to the Tetbury Road Station of the Great 

 Western Railway, North Wilts ; the small example is from the Cornbrash of Hilperton, 

 which has also produced other specimens. There are several in the British Museum, of 

 medium sizefrom the Great Oolite of Ranville, Normandy. This appears to have affinity 

 much larger and imperfect fossil figured by Goldfuss (' Petrefac.,' pi. 136, fig. 5 c) 

 from the Lower Oolite of Pegnitz, forming one of a group which he attributed to 

 T. literata. Its nearest ally probably is T. wididata, which is obtained in a similar 

 geological position ; compared with the latter species it has greater convexity, more 

 especially at the umbonal portion of the valves, which has also the costae mucli more 

 closely arranged, more horizontal, more numerous, and more rounded ; the slight posteal 

 flexure downwards and the considerable angles which they form with the short, nodulous,, 

 perpendicular series is also distinctive. In T. undulata the angle or curvature is situated 

 nearly at the middle of the valve. 



Messrs. Terquem and Jourdy have figured a large example of T. detrita in their 

 ' Monograph on the Great Oolite of the Department of the Moselle,' 



Trigonia Clytia, D'Orb. PI. XI, figs. 4, 5 ; PI. XVII, fig. 7. 



Thigonia Clvtia, jyOrhigny. Prodrome de Paleont, 1850, vol. i, p. 309. 



— — Lycett. Gr. Ool. Suppl. Monog., Pal. See, 1863, p. 48, pi. xl,. 



fig. 5. 



— — Rigaux et Sauvage. Descr. de quelq. esp. nouv. de I'Etage Batbo- 



nien du Bas-Boulon., 18G8, p. 19. 



Shell small, sub-trigonal, moderately convex ; umbones elevated, sub-mesial, pointed, 

 and recurved ; anterior side produced and curved elHptically with the lower border ; 

 superior border short, somewhat convex, passing abruptly downwards to the pointed 

 posteal extremity. Escutcheon small and depressed. Area narrow ; the plane of its 

 surface forms nearly a right angle with the other portion of the shell ; it is traversed 

 transversely by a few large depressed costellae, which are waved as they pass the mesial 

 furrow ; there are also three slightly nodulous carince, of which the inner and median 

 carinae are very small, the marginal carina is much larger. The other portion of the shell 

 has the costae plain, numerous, small, and very closely arranged, convex upon their lower 



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