﻿COSTATiE. 



157 



downwards ; its length is much greater than that of the siphonal border ; its surface has 

 rugose, irregular, oblique, depressed, large-knotted costellse ; they appear to be variable 

 in character. The interior of the valves have the dental hinge-processes unusually large 

 and prominent. 



Positions and Localities. This large variety has been obtained only in the Cornbrash 

 of the north of England ; the large valve, fig. 4, is from Southern Lincolnshire, the 

 others are from the vicinity of Scarborough ; occasionally specimens in the shortness of 

 their costated surfaces approach to the variety angustata, but usually the two forms are 

 sufficiently distinct. 



The lines of growth are conspicuous upon well-preserved examples of all the 

 varieties; when they are of fully developed growth the lines replace all the surface- 

 ornaments. 



To the Weymouth or typical form apparently should be assigned a Trigonia, which 

 occurs in the Elsworth Rock of Cambridgeshire, examples of which have been forwarded 

 to me by Mr. J. E. Walker ; their condition of preservation is indifferent. 



Trigonia cardissa, Agassiz, so well delineated in the work of that author (' Trigonies/ 

 tab. xi, figs. 4 — 7), should be arranged as distinct from T. elongata. There is much 

 general neatness in the surface-ornaments ; the escutcheon is depressed ; the marginal 

 carina is comparatively small ; the costse are narrow, somewhat oblique, and curved 

 almost perpendicularly upwards upon the anterior face of the shell, which forms a 

 considerable excavation ; this last feature in the eostae separates it decisively from the 

 British group allied to it. Agassiz did not ascertain the stratigraphical position of 

 T. cardissa j both Quenstedt and Oppel refer it to the Kelloway Rock of Erance and 

 Switzerland. D'Orbigny ( { Prodrome de Paleont.,' vol. i, p. 338, No. 161) makes T. 

 cardissa a synonym of T. elongata, but excludes figures 1 and 2 of the 'Mineral 

 Conchology/ which are Weymouth specimens, and unites them to the Neocomian 

 T. carinata of Agassiz. These arrangements were made in the absence of sufficient 

 knowledge of British species. T. cardissa is not known as a British species. 



Trigonia sculpta, Lyc. Plate XXXIV, figs. 1, 2, 2 a. 



— — id., var. Cheltensis, fig. 3. 



— — ib., var. Bolandi, fig. 4. 



Trigonia costata, Knorr. Versteinerungen, Supplement, tab. v c, figs. 3, 4, 1772. 



— — Smith. Strata Identified, Cornbrash, fig. 4, 1816. (Var. Rolandi, 



Cross.) 



— — Deshayes. Encycl. Method., Suppl., tab. ccxxxviii, fig. I, a, b, 



1836, 1838. 



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