﻿126 



BRITISH FOSSIL TRIGONI/E. 



Affinities, and Stratigraphical position. — The first few specimens examined were very 

 imperfectly preserved ; they were obtained by Mr. C. J. A. Meyer in beds of Chloritic 

 Marl and Greensand at Dunscomb Cliffs, to the eastward of Sidmouth, and were at first 

 regarded as dwarfed representatives of T. abrupta, Von Buch, figured by Coquand 

 (' Monogr. de l'Etage Aptien de l'Espagne/ pi. xiii, figs. 4, 5), apparently from a large 

 and fine specimen, nearly three inches in length and more than half that measurement 

 through the united valves ; it is, therefore, two and a half times larger in linear dimen- 

 sions than British examples of its analogue ; it also presents some differences in figure ; 

 its umbones are much more nearly erect, the anterior border is less rounded, the outline 

 of the siphonal border is much more oblique and less short ; the contrast between the 

 anteal, horizontal, inflated costse, and the posteal, smaller, and perpendicular ones, is 

 much less conspicuous ; but, perhaps, the most marked distinction consists in the absence 

 of the numerous transverse costellse upon the anteal half of the area. As the Spanish 

 specimen is from the Aptian beds, we should scarcely expect to find identity of species in 

 fauna so widely separated, both geographically and stratigraphically. The drawing of 

 T. abrupta, Von Buch (' Petref. recueil. en Amer. par Alex, de Humboldt et par 

 Ch. Degenhardt/ fig. 21), to which Coquand referred his specimen, is defective in the 

 several details which are necessary to enable us to characterise Cretaceous Trigonia ; the 

 general figure also differs so decidedly from all known European examples that, in 

 the absence of M. Coquand's monograph I should not have ventured to allude to the 

 South- American fossil as a form nearly allied to that in the higher beds of the Sidmouth 

 Upper Greensand. The subovate outline especially differs ; the shell appears to be without 

 the anteal inflation and posteal attenuation common to several species of the aliformis 

 group; the umbones without elevation or curvature, the straightness of the divisional angle of 

 the valve, the absence of all character upon the area and escutcheon, and even the 

 drawing of the costae upon the side of the valve, cannot be accepted as delineations of 

 the European Aptian form, and still less of the smaller British species ; the curvature 

 of the anteal costge has little of the angularity and abruptness upon which the name is 

 founded. The name was probably retained by Coquand in the belief that the American 

 specimen figured by Von Buch is very defective in the several features above mentioned, 

 and that the species is probably identical with the Spanish Aptian Trigonia. Another 

 interesting South-American Trigonia figured by Von Buch is T. Humbotdtii, figs. 29, 30, 

 which has costse radiating from the umbo over the greater portion of the surface ; it has 

 affinities with T. divaricata, D'Orbigny, and tends to connect the Cretaceous Scabrce with 

 the living Australian section, the Pectinida. 



At several localities near Sidmouth our species is associated with T. laviuscula, 

 T. sulcataria, T. pennata, and T. Vicaryana ; it occurs only within a very limited vertical 

 range; several better preserved specimens, including the original of the figure 

 (Plate XXIII, fig. 6), have proved its distinctness from the Aptian form. Other 

 specimens from Chardstock are in the Museum of the Royal School of Mines ; only 



