﻿GLABRAE. 



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Shell subovate, very convex; umbones prominent, obtuse, antero-mesial, much 

 incurved and slighly recurved ; anterior side short, curved elliptically with the lower 

 border; hinge-border short, convex, curving downwards posteally with the siphonal 

 border, the length of which nearly equals that of the hinge-border. Area narrow, its 

 surface forming nearly a right angle with the other portion of the valve ; it is slightly 

 concave near to the apex, becoming convex posteally ; it is divided by a deeply marked 

 mesial furrow, and is traversed transversely in common with the whole shell by delicate 

 lines of growth ; it is bounded by two small carinse, which are conspicuous near to the 

 apex; the marginal carina has a few small distantly arranged tubercles; the inner 

 carina is also slightly knotted. The escutcheon is small, depressed, but becomes some- 

 what elevated at its upper border. The ante-carinal space is remarkable for its great 

 breadth, which at the pallial border exceeds that of the area, and is equal to one third of 

 the length across the valve, its upper portion forms a considerable concavity. The 

 anteal or costated portion of the shell is comparatively narrow, occupying only half the 

 surface of the valve ; the costae are plain, oblique, and have some irregularity, curving 

 downwards from the anterior border, and terminating abruptly at the smooth and more 

 depressed ante-carinal space ; for the most part their posteal extremities become 

 irregularly nodose. An arrest of growth or concentric sulcation occurs beneath the 

 middle of the valve ; the costse subsequently have less obliquity, or are more concentric, 

 curving upwards anteally, and externally to the extremities of the costse upon the upper 

 half of the valve. 



The large proportion which the smooth ante-carinal space bears to the other portions 

 of the surface, together with the few, plain, oblique, and irregular series of short anteal 

 costse, constitute the most conspicuous distinctive characters. 



Our figure is taken from a gutta-percha pressing of an external cast in the 

 collection of Mr. Cunnington, and was obtained by him in the Portland Oolite 

 of the vicinity of Devizes ; a second specimen in a condition nearly similar is in 

 the same collection. Specimens of the typical form of T. Michelotti from the Kim- 

 meridge strata of Boulogne were figured by Goldfuss under the name of Lyrodon 

 excentricum, and by De Loriol under that of Trigotiia Michelotti; it was also 

 described and stratigraphically determined by Professor Hebert under the name of 

 T. Munieri ; it has a lower position than that of our Devizes specimens ; the Boulogne 

 shell also possesses some conspicuous distinctive features ; the figure is more lengthened, 

 the umbones are much less elevated, and less recurved ; the short anteal costse are less 

 prominent, or are somewhat obscure, and are therefore not nodose ; the carinas have less 

 distinctness, and are without tubercles. These differences are of considerable importance, 

 and were they founded upon a sufficient number of specimens, both French and British, 

 there would remain no doubt of the propriety of separating them as species, but with the 

 very limited materials of either form at my disposal or brought under my notice, and 

 knowing the variability exhibited by some of the Trigonice Glabra of the Portland 



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