TEREBRATULINA. 39 



Terebratula gracilis, Roemer. 1840. Die Vers. Norddeutschen Kreidgebirges, p. 40, 



No. 27. 



— — V. Hagenow. Mon. der Rug. Kreid. Vers., 1842. 



— rigida and gracilis, Morris. Catalogue, 1843. 



— gracilis, Geinitz. Grundriss der Verst., pi. xxi, fig. 10, 1844. 

 Terebratulina gracilis, D'Orbigny. Geol. of Russia and Ural, vol. ii, p. 499, 



pi. xliii, figs. 24—26, 1845. 

 Terebratula gracilis, Reuss. Die Vers, der Bohemischen Kreideformation, p. 49, 



pi. xxvi, fig. 1, 1846. 

 Terebratulina gracilis, TfOrbigny. Pal. Francaise Ter. Cre'tacees, vol. iv, p. 61, 



pi. 503, figs. 1—6, 1847. 

 Terebratula gracilis, Bronn. Index Pal., p. 1237, 1848. 

 Terebratulina gracilis, If Orb. Prodrome, vol. ii, p. 198, 1850. 

 Terebratula gracilis, C. Puggaard. (Ge'ol. de l'lle de Mben.) Bull, de la Soc. Ge'ol. 



de Fr., vol. vii, p. 534, 1850. 



Diagnosis. Shell inequivalve, orbicular, circular or elongated, either wider than long 

 or longer than wide ; larger or dental valve always convex ; the smaller one either mode- 

 rately so, or flat, never gibbous ; beak not much produced ; foramen small, formed 

 partly out of the substance of the beak, and by two small lateral deltidial plates, the 

 anterior portion being completed by the umbo ; beak ridges well defined, leaving between 

 them and the hinge line a fiat space or false area ; auricles on either side of the umbo 

 rather small ; valves ornamented by a variable number of radiating elevated stria?, often 

 granulated, which, soon after leaving the beak and umbo, augment rapidly in number, 

 rarely from bifurcation, but almost always by the intercalation of smaller costae, at variable 

 distances between the larger ones, and extending to the front and lateral margins; 

 concentric lines of growth more or less prominent ; structure punctuated ; internal 

 calcareous supports short and anneliform. Dimensions variable : length 5 £ ; width 5 ; 

 depth 2 lines. The comparative depth in some specimens is greater than is here given. 



06s. Schlotheim, in 1813, appears to have been the first author who noticed and 

 figured this species, stating it to occur in England ; and it is the same as that described 

 later, from the Chalk near Norwich, by Sowerby, under the appellation of Ter. rigida. It 

 appears, likewise, to have been one of those fortunate forms, to which the generality of 

 authors have applied the original name, notwithstanding its great variability in the con- 

 vexity of its valves and the number of its plaits, the last varying from eighteen to fifty on 

 each valve, round the edge. The costge, also, are not of equal width and depth, as almost 

 invariably, between two larger ones, one, two, or three smaller costse intervene, appearing 

 at different distances from the beaks, and extending to the front. It is, therefore, owing 

 to the greater or less number of these intervening plaits that the shell becomes more 

 closely or widely striated. It does not, however, present as much difference in the young 

 state as we discover in T. striata, and from this last is always easily distinguished by its 

 much more circular form and greater comparative width, as well as by the general aspect of 

 the shell. 



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