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BRITISH FOSSIL TRIGONE. 



costae resembles a similar feature in T. semiculta, Forbes, from the Cretaceous rocks of 

 Verdachellum, near Pondicherry, Southern India (see the description of T. aliformis) ; in 

 the Indian species the costge are much larger, and the posteal slope or area forms a 

 greater angle with the other portion of the valve. 



A similar feature is also conspicuous in T. Sancta Cruets, Valang, (Pictet, Paleon. 

 Suisse,' plate 128, figs. 1 — 5). The latter shell has the anterior side shorter and its 

 umbones more obtuse ; the anteal portions of its costse are also less distinctly horizontal 

 or excentrical. 



For T. Coquandiana, D'Orb., which is also an allied form, see the next species. 



Owing to the fragility of the test, and the more compact matrix, specimens in Green- 

 sand collections are usually very imperfect, and afford no adequate means for testing the 

 distinctness or affinity of other examples of the Glabra from the same formation. These 

 remarks also apply to specimens upon the tablets in the two national metropolitan 

 museums, and enhance the value of the aid which has been afforded by the contribution 

 from the collection of Mr. Vicary. 



As both T. excentrica and T. sinuata are figured upon the same plate in the ' Organic 

 Remains ' by Parkinson, neither form possesses priority ; I have made T. sinuata a 

 synonym, as it exemplifies only the very young condition of the more fully developed 

 T. excentrica. The internal mould does not appear to have been identified. The valves 

 are always disunited. 



Stratigraphical Positions and Localities. — T. excentrica in its different aspects occurs 

 in the Greensand of the Blackdown and Haldon regions at several localities, as at 

 Hembury Fort, at Staple Hill, and near to Collumpton. The Chloritic Marls and 

 Sandstones at Dunscomb Cliffs, to the eastward of Sid mouth, is another locality. 



D'Orbigny records T. sinuata, including T. qffinis, in the lower beds of his Terrain 

 Turonien or Chloritic Chalk of the Ligerian and Pyrenean basins ; the localities given 

 by him are Mans, Saint-Calais, Coudrecieux (Sarthe), Fouras, and the Isle d'Aix 

 (Charente Inferieure), Ambillon (Marne et Loire). He retained T. excentrica as distinct 

 from T. sinuata, but the only locality attached to it is Blackdown. 



Trigonia ljtviuscula, Lye., sp. nov. Plate XXII, fig. 6. 



Shell depressed, lengthened; umbo moderately produced, small, placed upon the 

 boundary line of the anteal third of the valve and slightly recurved ; anterior and lower 

 borders rounded elliptically ; superior border somewhat concave ; posteal extremity of 

 the valve produced, attenuated, and depressed, its outline rounded. The portion of the 

 valve adjacent to the posteal or superior border is slightly convex, and is without any 



