GENUS SPIRIFERA. 371 



SPIRIFERA PLICATELLA, var. RADIATA. 



PLATE 13 (4), FIGS. 9, 10 & 11. 



Spirifera pHcatella, var. radiata : Sow., Delthyris lineatus (text) , radiatus (index): 



Sow. Min. Con., Vol. v, p. 493, figs. 1, 2, 182-5 : not Anomia lineatus, Martin. 



Spirifer radiatus : J, DeC, Sow.; Silurian System, pi. xii, fig. 6. 1830. 



" " M'CoY, Sjmopsis of the Silurian fossils of Ireland, p. 37. 1848. 



" plicalellus : Salter, Memoirs of the Geol. Survey of Great Britain, Vol ii, 



p. 328. 1848. 



Spirifer cyrtcena : Davidson, Bull. Soc. Geol. de France, 2d series, Vol. v, p. 324. 



1848. 



Spirifer radiatus : Sow., Hall, Palajontology of N. Y., Vol. ii, p. GG and 265, pi. 



xxii, fig. 3, and pi. liv, fig. 6. 1852. 



Spirifer plicatellus, var. radiatus : Salter, Siluria, pi. ix, fig. 12, and pi. xxii, fig 7. 



1859. 



Spirifera plicatella : Lindstrom, Proceedings of the Royal Academy of Science of 



Stockholm, p' 358. 1860. 



It seems not worth while to risk the addition of another synonyme for 

 any variety of this very variable species, but the Wisconsin specimens 

 commonly referred to it present some unusual characteristics. The form is 

 rhomboidal or transversely oval, and usually very gibbous. The cardinal 

 extremities are rounded and the area more or less distinctly defined. Some 

 remains of surface striae are sometimes distinguishable on the cast, and 

 this marking is often well preserved in the matrix from which the shell has 

 been dissolved. Sometimes the margins of the valves, or their impression 

 in the cast, are undulated, and there are obscure remains of broad low 

 plications, which usually extend but a short distance, though sometimes 

 continuing nearly to the beak. The latter forms may perhaps be regarded 

 as S. plicatella proper : such specimens are very gibbous, with a high area 

 and broad deep sinus in the ventral valve, while they differ in form from 

 the simply striated specimens. 



The peculiarity noticed in the smooth or finely striated species is the 

 presence of distinct lamellae in the dorsal valve (as ^hown in fig. 9 of plate 

 13 (4) ), diverging from the apex and presenting all the characters of the 

 dental lamellae of the ventral valve. These marks upon the cast are not 

 simply sharp cut depressions, but the edges of distinct thin plates, which 

 are joined to the inside of the shell, sometimes for half its length. A speci- 

 men, fortunately broken, shows the interior of a shell without filling, and 

 these dorsal lamellae are seen extending downwards half way to the base, 

 and uniting with the shell precisely as the dental lamellae of the ventral 

 valve. These lamellae are divided near their origin, and give off the crura 

 from which the spires have continued. 



Specimens of this character are rhomboidal, gibbous, with distinct 

 mesial sinus and fold : small specimens, like the one figured, are more 

 common than larger ones, though they are sometimes found of much larger 

 size, and assuming a transversely oval form. From all the observations 

 made, it appears as though the dorsal lamellae were much stronger in the 



