GLABRiE. 83 



The area is narrow, slightly convex, having a mesial oblique furrow ; there are no distinct 

 bounding carina, but near to the umbo the area forms a distinct angle with the more 

 depressed ante-carinal space. The escutcheon is of moderate breadth, smooth and 

 depressed. The ante-carinal space is much depressed near to the umbo ; downwards it 

 becomes more flattened and widens regularly towards the lower border, where its breadth 

 exceeds that of the area. The entire valve in the adult state is divided into four or more 

 zones by large, deeply-indented, transverse sulcations which are always conspicuous ; they 

 curve upwards at their extremities in accordance with the lines of growth. The costated 

 portion occupies more than half the valve ; the costse in their prominence, number, and 

 general aspects possess so much variability that, without the possession of numerous 

 connecting specimens, other species may possibly be united with it ; whenever the costae 

 are distinct upon the first or umbonal zone they are moderately numerous, plain, and 

 oblique ; upon the next and the succeeding zones they have greater curvature ; anteally 

 the extremities of the costae in each zone curve upwards, external to the 

 extremities of the costae in the preceding zones, so that anteally the costae appear to be 

 unsymmetrical ; upon the last zone the costae become smaller and less distinct, or are 

 confused irregularly with the lines of growth. T. gihbosa may be arranged under three 

 varieties as follows : 



Var. a. — Figure, Unio like or produced posteally ; longitudinal sulcations large and deep, 

 irregular and unequal near the pallial border ; ante-carinal space narrow 

 and not well defined, excepting near the apex ; surface generally destitute 

 of ornamentation, occasionally with some indications of costae. Plate XVIII, 

 figs. 5, 6. 



Var. h. — Costae prominent and numerous, covering the greater portion of the valve ; 

 narrow, ridge-like, small, and plain anteally, forming large, oblong, or sub- 

 ovate nodes posteally ; ante-carinal space much larger and more defined than 

 in Var. a ; the area also more distinctly marked, sometimes with slightly 

 knotted elevations at the positions of the marginal and inner carinee. 

 Longitudinal sulcations distinct, but smaller than in Var. a. Plate XVIII, 

 figs. ], 2, 2 a, 4; Plate XIX, fig. 2. 



Var. c. — Ante-carinal space very large and depressed ; sulcations only slightly defined ; 

 rows of costae very numerous and irregular, with small, crowded, but 

 prominent nodes, producing a roughened surface ; area narrow, strongly 

 defined, transversely coarsely plicated, its bounding carinae knotted anteally, 

 plicated posteally. Plate XXI, fig. 1. 



The figures in Sowerby's ' Mineral Conchology ' represent the varieties a and 5. 



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