74 BRITISH FOSSIL TRIGONIJE. 



TrigonijE, or portions of them from the Portland formation of Oxfordshire and Wiltshire, 

 which have come under my notice, are too imperfect in their general condition to admit 

 of any satisfactory comparison with other known forms ; at present, therefore, I can only 

 allude to T. radiata as a British species with much hesitation, as being represented 

 possibly by examples which are too defective to be submitted to the artist as illustrations 

 of Miss Benett's species. It may also be remarked that an entire absence of ornamen- 

 tation upon the anteal portion of the shell, such as appears to be indicated by the 

 specimen of Miss Benett, by that of Munier-Chalmas, and also by the defective figure of 

 De Loriol, represents a feature to which we discover nothing analogous throughout the 

 entire series of the genus Trigonia. In the absence of any more satisfactory example it 

 is intended to figure an imperfect specimen in the Oxford University Museum, obtained 

 in the Portland Limestone of Shotover Hill, and kindly brought under my notice by 

 Professor Phillips. 



Trigonia tripartita, 'Forbes. PL XII, fig. 7. 



Tbigonia tripartita, Forbes. Quart. Jour. Geol. Soc, 1851, vol. vii, p. Ill, pi v, 



fig. 11. 



— — Morris. Catal., 1854, p. 229. 



— — Lycett. Pal. Soc. Suppl. Gr. Ool. Monogr., 1863, p. 51, p. xl, 



fig. 4. 



Shell ovately trigonal, short, rather depressed ; umbones elevated, obtuse, and 

 scarcely recurved ; anterior and lower borders curved elliptically ; hinge-border short, 

 sloping obhquely. A.rea of moderate breadth, flattened, distinctly bipartite, the inner or 

 superior half being the more depressed ; it is traversed transversely by delicate lines of 

 growth, and near to the umbones by a few small costellae ; the bounding carinse are distinct, 

 but have little prominence, and are only imperfectly tuberculated ; the posteal border of the 

 area forms only a slight angle with the superior border of the escutcheon, which is small 

 and depressed. The costated portion forms a considerable angle with the plane of the 

 area ; the costae form two distinct series ; the more numerous or anteal series has the 

 rows very small, plain, and closely arranged; their direction is uniform, passing from the 

 anteal border obliquely downwards to the middle of the valve, where they are cut ofi" by 

 a much larger, less numerous, posteal series of costse, seven or eight in number, which 

 are slightly tuberculated, and pass downwards from the carina almost perpendicularly, 

 forming right angles with the anteal series. The comparative size and direction of the 

 two species of costa? will serve to distinguish this little shell from T. v.-costata, from T. 

 undulata, and from T. detrita; others of the Undulatce are more remotely allied. The 

 anteal ribbing in Pig. 7 is more minute than in the specimen figured by Professor E. Forbes, 

 and this is the sole difference observable. 



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