TRIGONOSEMUS. 29 



Koenig, however, placed two other shells in his genus, which are now considered to 

 belong to different sections or genera ; Trigonosemus elegans must therefore be taken as the 

 type; later, in 1847, M. D'Orbigny applied to the same shell another generic title, that of 

 Fissurirostra} but which cannot hold priority over the name proposed by Kcenig many 

 years before. 



The arrangements of the loop in this shell are those of Terebratella, to which it bears 

 more affinity than to that of any other genus or subgenus, as admitted by M. D'Orbigny, 

 but the shape and character of its area, beak, and foramen, as well as the remarkably- 

 shaped boss and muscular impressions, seem to entitle it to sub-generic distinction, since 

 differences, much less important, are very often made use of in the separation of genera in 

 other classes of Mollusca. In some of the species, such as Trigonosemus pulchella and 

 pectiniformis, the foramen is so very minute, as to be generally visible only by the lens, 

 which makes M. D'Orbigny suppose, that in every adult specimen the aperture became 

 obliterated, and that the animal lived with its smaller valve upwards, and the larger one 

 under, — the reverse of what exists in Terebratula. I am not, however, convinced of the 

 correctness of this observation, from having a number of specimens of both the above 

 species before me, in all of which the foramen, although small, is perceptible ; and, owing 

 to the incurved beak, we cannot believe it was attached differently from what we observe 

 in Terebratula. The sub-genus Trigonosemus seems at present peculiar to the Cretaceous 

 period. 



10. Trigonosemus elegans, Koenig. Plate IV, figs. 1 — 4. 



Trigonosemus elegans, Kcenig. Icones Fossilium Sectiles, p. 3, pi. vi, fig. 73, 1825. 

 Terebratula elegans, Def ranee. Die. d'Hist. Nat., vol. 53, p. 157, 1828. 



— recurva, „ Die, p. 161, 1828. 



Fissurirostra recurva, If Orb. Pal. Franc. Ter. Cretacees, vol. iv, p. 133, pi. 520, 



figs. 1—8, 1847. 

 — elegans. lb., vol. iv. p. 135, pi. 520, figs. 9—13, 1847. 



Terebratula elegans, Bronn. Index Pal., p. 1236, 1848. 



— recurva. lb., p. 1248. 

 Fissurirostra elegans and recurva, If Orb. Prodrome, vol. ii, p. 259, 1850. 



Diagnosis. Shell inequivalve, irregularly oval, or somewhat rhomboidal, generally 

 longer than wide ; valves unequally convex, the greatest width and depth towards the 

 middle ; dental valve very convex longitudinally, keeled ; beak large, much produced, 

 moderately recurved, and obtusely truncated by a small, narrow, elongated, oval foramen 

 or fissure, through which the pedicle issued ; area very large, triangular, wider than long, 



1 'Considerations Zoologiques et Ge'ologiques sur les Brach.,' * Comptes Rendus Hebdomadaires de 

 I'Academie des Sciences,' 1847, and 'Pal. Francaise Ter. Cretacees,' vol. iv, p. 132. 



