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BRITISH FOSSIL TRIGONLE. 



specimen figure 2, so that when placed horizontally and viewed from above, no portion of 

 the area is seen. The internal mould is not known. The hinge teeth are widely 

 divergent, they have little prominence, and are smaller than is usual with the larger 

 examples of the Costata. 



Position and Localities. Upon the Coast of Yorkshire to the southward of Scarborough 

 it is common in the bed of Cornbrash and also in the small exposure of the same bed 

 to the northward of the Castle Hill, where the specimens are badly preserved or have 

 suffered from compression — a condition which invariably occurs when the valves are in 

 contact. The species has also been obtained rarely in the brown, sandy, lowest bed of 

 Kelloway Rock in the same vicinity. The compression and distortion which is so 

 common in this large Trigonia is a frequently recurring feature in the larger testacea of 

 the Cornbrash ; the other large Trigonia? from the same bed and locality present similar 

 defects in their state of fossilisation. 



As the Trigoniae generally are good stratigraphical guides it will appear remarkable 

 that its representatives which characterise the Cornbrash or Forest Marble throughout the 

 southern and midland countries of England have altogether disappeared to the northward 

 of the Humber, where they are replaced by other species. Thus, upon the Coast of 

 Yorkshire we look in vain for T. pullus, T. costaia, T. Jlecta, T. undulata, var. arata, T. 

 Moretoni, and the short variety of T. sculpta designated Hoi audi ; these are replaced in 

 the northern Cornbrash by T. Scarbwyensis, T. C'assiope, and by two varieties of 

 T. elongata, all of which occur in some abundance. 



The changes thus exhibited by the genus Trigonia will be found scarcely less 

 remarkable in other associations of Molluscan forms, more especially in the numerous and 

 varied series of Conchifera, some of which are identical with Kelloway Rock species and are 

 unknown in the southern Cornbrash. These data tend to the conclusion that the Corn- 

 brash of Yorkshire represents a deposit of marine testacea more recent than that of the 

 southern countries or more transitional and tending to connect more nearly the mollusca 

 of the Lower and Middle Oolites. 1 



The foreign localities assigned to T. Cassiope are Luc (Calvados) ; Vezelay (Yonne) ; 

 Grange Henry rear Nantua ; it also occurs rarely in the Great Oolite of the Bas-Boulonnais 

 (Rigaux et Sauvage). 



Mr. R. (now Professor) Tate, ' Quart. Jour. Geol. Soc.,' 18G7, p. 171, records its 

 occurrence in rocks (supposed to be Jurassic) in Southern Africa. The single specimen 

 upon which this identification is founded has scarcely half the linear dimensions of York- 

 shire examples ; the distinctive characters are not prominent, and in the absence of more 

 satisfactory materials for comparison, the specific identity may be regarded as doubtful. 



1 See a note by the present author on the association of generic forms of Mollusca in the Yorkshire 

 Cornbrash compared with those of the Cornbrash of the southern counties, ' Supplementary Monograph 

 on the Mollusca of the Stonesfield Slate, Great Oolite, Forest Marble, and Cornbrash, Palaeontographicai 

 Society,' volume for the year 1861, p. 117. 18(33. 



