﻿20n BRITISH POSSIL TRTGONLE. 



Compared with T. Scard/oyensis, PI. IV, figs. 1 — 4 (which is also a very variable 

 sjjecies, almost limited stratigraphically to the bed of Cornbrash), the short example of T, 

 Rupelhnsis, PI. VIII, fig. 4, and other similar forms, show only remote alliance ; neither 

 does T. Bupdleniiis exhibit that considerable difference in the surface ornaments of opposite 

 valves of the same shell, which so commonly occurs in the Cornbrash form. Upon the 

 whole there is much affinity between the more lengthened forms of the two species ; and, 

 if we exclude from comparison the greater number of the left valves of T. Scarhurgensis 

 similar to the specimen, PI. IV, fig. 3, it will occasionally be found difficult to give any 

 definite distinction between the two forms. Generally it may be stated that the 

 variability in the costse, and also in the general figure of the valves, is much more 

 considerable in the Kelloway Rock form, and that none of the latter have the figure so 

 much lengthened and depressed posteally as the Cornbrash species ; the latter also have 

 the rows of costas usually more horizontal, and they approach the carina at a lesser angle 

 than in T. Txupellemis. The result of an ample comparison of specimens has been to 

 confirm the propriety of retaining the two allied forms as separated both by palasonto- 

 logical and stratigraphical distinctions. 



Tkigonia undulata, From., var. arata. Page 77, Plate XVI, figs. 9 — 11 ; Plate XVII, 



figs. 5, G. 



The British examples figured PI. XVI, figs. 9—11, and PI. XVII, figs. 5, 6, 

 described at p. 77, and alluded to p. 48 may be regarded as a variety of T. undulata, 

 figured by Agassiz (' Trigonies,' tab. 10, figs. 14 — 16) from the Great Oolite of 

 Piedmont. Other fine examples of the typical form have since been obtained in the 

 mountain district of the Lebanon to the eastward of the town of Beyrout ; specimens from 

 the latter locality have been known under the name of Trigonia Sgriaca. Compared 

 with the British variety arata, it has somewhat greater convexity upon the middle and 

 lunbonal portions of the valves ; the marginal carina has greater prominence and the 

 siphonal border is more lengthened or more oblique, thus shortening the length of the 

 hinge-border. As these differing features are very persistent, there can be no doubt of 

 the propriety of separating the British fossils as a variety when compared with the 

 typical form from Italy and from the Lebanon. All the latter specimens examined have 

 the last-formed costse scarcely developed. The subjoined engravings represent a Syrian 

 specimen of full dimensions. 



It may be a subject of doubt whether our British variety arata may not be fitly 

 separated from the continental or typical form, and constitute a distinct species. Our 

 Cornbrash and Great Oolite specimens possess much variability, and more than one of 

 them which have come under my notice are separated but little from the typical form in 

 the almost entire absence of tubercles upon the costae. 



