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BRITISH FOSSIL TRIGONLE. 



Affinities and Differences. The Lyrodon sulcatum of Goldfuss is one of the Cretaceous 

 Scabrce, and altogether a different species ; it is better known as Trigonia sulcataria, to 

 which the reader is referred. T. peninsularis, Coquand (' Monogr. de l'etage Aptien de 

 PEspagne,' pi. xxiii, fig. 3), is the only other costated Trigonia known in the Cretaceous 

 rocks ; its general aspect is that of sectional degeneracy, and has no near affinity with 

 T. carinata ; it is chiefly remarkable for the very irregular and unequal costse, some of 

 which are angulated mesially ; the few last formed become irregular plications. 



By a singular error, which could only have arisen from an imperfect acquaintance with 

 British Trigonia, D'Orbigny ( £ Prodr. de Paleont.,' vol. i, p. 338, No. 161; also 

 vol, ii, p. 78, No. 288) made Sowerby's figures 1, 2, of T. elongata, from the Oxford 

 Clay of Weymouth, to be identical with the Neocomian T. carinata. Our figures of each 

 of these species render any detailed comparison between them unnecessary. 



Positions and Localities. T. carinata occurs in the Upper Neocomian formation, in 

 the lowest or Perna Mulleti bed at the Atherfield section in the Isle of Wight ; the test is 

 usually only partially preserved ; specimens are of every stage of growth, but it is some- 

 what rare, and its condition of preservation compares unfavorably with the numerous 

 valves of Conchifera at that celebrated locality ; other localities and Neocomian positions 

 are Hythe, Lympne, and Maidstone, at which places the test is usually absent, the fossil 

 consisting of glauconitic sandstone. It is unknown in the Middle Neocomian Stage at 

 Speeton, Tealby, Norfolk, and Cambridgeshire. 



Our figures represent Atherfield specimens excepting Plate XXXV, fig. 3, from 

 Upper Greensand, near to Ventnor. The latter, although deprived of the test, has the 

 surface ornaments well preserved. Apparently also the range of T. carinata is even more 

 considerable than is indicated by the Isle of Wight specimens. Certain internal moulds, 

 very imperfect and unfit to be submitted to the artist, were forwarded to me by Mr. 

 Cunnington from the Upper Greensand of Devizes and from the Chloritic Marls of 

 Warminster ; their occurrence at the latter locality and position in the highest beds of 

 the glauconitic series extends the range of T. carinata, and exceeds that of any other 

 known example of the genus. 



It is also noteworthy that, although the range both stratigraphically and geographically 

 is so considerable, no separation into varieties occur ; the more close or more distant 

 arrangement of the costae represent the limits of variation. It will be observed that the 

 small specimen from Upper Greensand of Ventnor, Plate XXXV, fig. 3, offers no 

 distinction from the specimen, fig. 4, which is of similar size, and obtained in the lowest 

 bed of the Neocomian formation at the Atherfield section, separated from the newer 

 position by upwards of 1000 feet of strata. 



T. carinata is absent in the Gault both of Kent and of the Isle of Wight, and 

 equally so in the Greensand of the Blackdown and Haldon regions and in the Chloritic 

 Marls of the South Devon Coast. 



Foreign localities cited are all Neocomian ; these are Saint Saveur, Vaux-sur-Blaize, 

 Brillon, Greaux, Vorey, near Besancon. Switzerland — Hautervive, near Neuchatel. 



