﻿186 



BRITISH FOSSIL TRIGONLE. 



the siphonal border, the superior extremity of which forms a similar angle with the 

 posteal extremity of the lengthened, narrow, elliptical escutcheon. The area is somewhat 

 convex, its size is nearly equal to the other portion of the valve ; it has a series of large 

 acute transverse costellae, which are somewhat broken at the position of the usual median 

 furrow, which is not distinct. The marginal carina is represented by a row of regular, 

 minute, rounded papillae. The other or pallial portion of the valve has a small series of 

 subnodose varices which are variable in figure ; occasionally the anteal varices are curved 

 and entire, more frequently they are broken mesially, when their anteal portions become 

 horizontal ; posteally there are two or three short perpendicular rows. The interiors of 

 the valves have the nacreous layer of the test preserved ; the hinge processes are unusually 

 large ; the borders of the valves are plain excepting the depressions and prominences near 

 the posteal extremity of the pallial border. 



The anteal truncation and the characters of the varices ally it to the Scaphoidea ; 

 the internal pits near to the posteal extremity of the pallial border resemble a similar 

 feature in the Cretaceous Quadrata, from which section, however, it is separated decisively 

 by its plain escutcheon. 



The T. pulchetta of Reuss ( c Die Versteinerungen der Bohmischen Kreideformation/ 

 tab. 41, fig. 3) is a minute form, having no affinities with the species to which Agassiz 

 had previously given the same name ; only two lines in length, it is allied to and may 

 possibly represent the very young condition of T. disparilis, D'Orb. ( c Pal. Fran. Terr. 

 Cret.,' vol. 3, plate 299), which it resembles in its radiating, knotted, unequal costa?, and 

 in the transverse costellae which cross both the area and escutcheon ; it has not occurred 

 in Britain. 



The stratigraphical range of Trigonia pidchella, Ag., appears not to be limited to a 

 single geological position ; it was first obtained by M. Gressly at Urweiler and at 

 Muhlhausen (Department of the Haut-Rhin), in beds which were assigned by himself 

 and by Agassiz to Upper Lias, but regarded by them as representing a peculiar and local 

 development of that formation. 



M. Terquem obtained this Trigonia in the bed of Marly Sandstone or Gres Supra- 

 liassique of the Department of the Moselle, associated with Trigonia navis, T. litterata, 

 and a series of Testacea, several of which have their equivalents in some portion of the 

 Supra-liassic Sands of England, in the Cotteswold Hills, and in the southern counties. 

 The more northern or Yorkshire development of these sands presents differences both 

 lithological and palscontological. 



Professor Qucnstedt from an extensive knowledge of the strata of Urweiler and its 

 numerous Testacea exemplified in plates 42, 43 of his ' Der Jura,' determined the 

 position of T pulchella to be the lowest zone of Inferior Oolite or that of Ammonites 

 torulosus, associated with Trigonia navis, Ammonites opalinus, A. Hircinus, Venulites 

 trigonellaris, Trigonia similis, &c, a remarkable association of Testacea which is almost 

 wholly distinct from any British assemblage of Inferior Oolite fossils. Several of these 



