﻿50 BRITISH FOSSIL TRIGONI^E. 



spicuous mesial furrow; the transverse plications of the area are small and closely 

 arranged, excepting upon its posteal third, where they become large, rugose^ irregular, 

 forming some varices at the hmer carina. The escutcheon is depressed, of moderate 

 breadth and great length ; its upper or inner border is raised. 



The other portion of the shell is occupied by a series of luunerous and closely arranged 

 costaj, twenty-two in number; they are nearly of equal size, rounded, more or less 

 knobbed or transversely plicated, and are only slightly attenuated near to the marginal 

 carina ; their general direction is oblique, or at right angles to the marginal carina, but 

 the seven last formed or posteal costse are nearly perpendicular ; for the most part they 

 are somewhat undulated, or even wrinkled, and there is one supplementary or intercalated 

 costa upon the anterior side. The lines of growth are distinct over the greater portion 

 of the valve. The small tubercles upon the costae are more distinctly traced upon the few 

 umbonal or more concentric ones, upon the succeeding costae they are very unequal and 

 frequently indistinct. 



The length of the marginal carina is equal to nearly twice the height and to three 

 times the diameter through both the valves ; the general aspect is sufficiently distinctive, 

 and does not approximate very nearly to any other species of the Lower Oolites. 

 T. signata is more produced anteally and more truncated posteally ; the upper portions 

 of its costae are more attenuated, they also approach the carina at a much greater 

 angle. 



Geological position and locality. This rare Trigonia has only been obtained in the 

 Ammonite-bed of the Supra-Liassic Sands ^ at Frocester Hill. The specimen in the 

 National Museum, Jermyn Street, and another in the cabinet of Dr. Wright at Cheltenham, 

 are the only examples with which I am acquainted. 



Trigonia muricata, Goldf. Plate IX, fig. 1. 



Lyrodon mhricatum, Goldfuss. Petref. German., 1836, tab. 13", fig. 1, p. 201. 

 Teigonia MUKICATA, iJoeme/. Nordd.Ool. Nachtrag., 1839, p. 75. 



— — Agassiz. Trigouies, 1840, pp. 7 and 51. 



— — lyOrbigny. Prodrome de Paleont., 1850, vol. ii, p. 51, No. 120. 



— — Oppel. Juraformation, 1857, p. 719, No. 89. 



1 I prefer provisionally to employ this term to designate the sands or marly sandstones which separate 

 the Inferior Oolite from the Upper Lias {Am. communis Zone) over such distant areas in England, 

 France, and Germany. After collecting for many years and comparing the faunas of these several zones, 

 the conclusion has long been present to my mind that the fossils of these sands, viewed as a whole, are 

 clearly separable from those of the beds both anterior and posterior to them in chronological order ; tlie 

 boundary lines of this stage, both lower and upper, are neither abrupt lithologically nor palseontologically, 

 each has its species of testacea whose limits vertically may extend to one or both sides of our convenient, 

 but somewhat arbitrary, divisional lines; nevertheless its fauna, viewed as a whole, is well marked and 

 characteristic of the stage. 



