SPIRIFERA. 51 



Spirifeea pinguis, Sow. Ibid., figs, 18, 19. 



— — Fan Buck. Ueber Delthyris, p. 38, 1837, and Mem. Soc. G^ol. de 



France, vol. iv, p. 184, pi. 8, fig. 7- 



— KOTUNDATUS, Sow. De Koninck, Animaux Fossiles de la Belgique, p. 263, 



pi. xiv, fig. 2, and pi. xvii, fig. 4, 1843. 



— PINGUIS, Sow. Ibid, p. 661, pi. Ivi, fig. 5, 1851. 



— — Morris. Catalogue, p. 153, 1854. 



— — M'Coy. British Palaeozoic Fossils, p. 420, 1855. 



— SUBROTUNDATA, M'Coy. Ibid., p. 423, 1855. 



Spec. Char. Very variable in shape, dimensions, relative proportions, and degree of 

 convexity. When full grown (under favorable circumstances) the shell is transversely 

 oblong or oval. Sometimes as wide as long, very rarely longer than wide ; hinge line 

 rather shorter than the greatest width of the shell ; cardinal angles rounded. Dorsal 

 valve not so convex or deep as the opposite one, mesial fold wide, moderately raised, 

 almost smooth, and divided along its middle by a shallow longitudinal depression or 

 groove. Each valve is ornamented by from sixteen to thirty rounded or flattened ribs, 

 which sometimes vary in their relative widths, a small rib being situated side to side by a 

 larger one ; they are also regular and simple in the larger number of individuals, but in 

 some a few bifurcate or become more numerous by occasional intercalations. A narrow 

 hinge area is likewise observable in this valve. 



Dental or ventral valve at times very gibbous, beak of moderate dimensions, much 

 incurved ; area narrow, with a triangular fissure partially covered by a pseudo-deltidium. 

 A sinus of greater or lesser depth extends from the extremity of the beak to the front, 

 and is in general ornamented by two or more narrow longitudinal ribs of but small 

 elevation. The spiral appendages are large, and occupy the greater part of the interior of 

 the shell. 



The relative proportions vary considerably, as will be perceived from the measurements 

 taken from four adult individuals : 



Length 2 inches 7 lines, width 3 inches 8 lines, depth 2 inches. 



„ 4 „ 4 „ 2 „ 6 „ 1 „ 9 lines. 



1 „ 6 „ 1 „ 5^ „ 1 „ 4 lines. 



Obs. How to deal with the innumerable variations in shape and proportions 

 presented by this species seems to have been no easy matter. Sowerby and Phillips, 

 while describing Sp. pinguis and 8p. rotundata, Sowerby, as distinct species, did not fail 

 to observe that both were nearly allied forms, and that Sp. pinguis may be the young 

 state or a simple variety of Sp. rotwndata., Sowerby. In his work on the Belgian fossils. 

 Professor de Koninck describes both as separate species, but now relinquishes that idea 

 and is of opinion that they should be combined under a single denomination. The 

 illustrations I have selected for Plate X demonstrate in the most incontestable manner not 

 only the affinity existing between these two shapes, but their complete specific identity, an 



