176 CARBONIFEROUS LAMELLIBRANCHIATA. 



Non Nucula, B. Etlieridge,jun., 1876. Ann. and Mag. Nat. Hist., ser. 4, vol. xviii, 



p. 104. 



— — 1878. Ibid., ser. 5, vol. ii, p. 33. 



— Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 302. 



— White, 1879. Bull. U. S. Geol. Surv. Territories, vol. v, No. 2, p. 216. 

 Ctenodonta, Barrois, 1882. Asturias et Galice, p. 339. 



Nucula, Walcott, 1884. Monogr. TJ. S. Geol. Surv., Pal. Eureka District, 



p. 241. 



— de Koninck, 1885. Ann. Musee Eoy. Hist. Nat. Belgique, vol. xi, 



p. 132. 



— Etheridge, 1888. Brit. Toss., pt. 1, Palaeozoic, p. 287. 



— S. A. Miller, 1889. N. American Geol. Palaeontol., p. 494. 



— A. H. Worthen, 1890. Geol. Surv. Illinois, vol. viii, p. 128. 

 Non — G. Wild, 1892. Trans. Geol. Soc. Manch., vol. xsi, p. 396. 



Goodchild, 1892. Proc. Eoy. Phys. Soc. Edin., vol. xi, p. 244. 



— Keyes, 1894. Geol. Surv. Missouri, vol. vi, pt. 2, p. 121. 



? Ctenodonta, Tornquist, 1896. Fossilfuhr. Untercarbon. Sudvogesen, p. 74. 



Generic Characters. — Shell equivalve, inequilateral, close all round, more or less 

 gibbose. Hinge angular, multidenticulate, hinge-teeth in two sets separated by a 

 small fossa for the ligament, which is internal. Adductor impressions well 

 marked. Pallial line entire. 



Observations. — This genus, easily recognised whenever its peculiar hinge 

 characters have been exposed, has not suffered as much as most of the other 

 Carboniferous genera of Lamellibranchs at the hands of palaeontologists. The 

 earlier authors generally placed together under the genus "Nucula" those shells 

 now recognised as Nucula and Nuculana, but Salter referred the Carboniferous 

 examples of both of these genera to his genus Ctenodonta. 



Mr. Goodchild (pp. cit.) proposes to return to the former nomenclature, 

 because, as he correctly observes, the Carboniferous shells known as Nuculana 

 (Leda) do not possess a sinuated pallial sinus. But, on the other hand, they differ 

 from the shells known as Nucula, in being more transverse, and also possessing an 

 attenuated and pointed posterior end, an oblique more or less marked keel, and a 

 well-marked escutcheon and lunule. In addition the Carboniferous Nucidanee 

 have a curious prominence in the centre of the umbonal hollow, which is seen in 

 casts as a vertical groove, causing the cast of the umbo to appear bifid, which 

 character is absent in the Carboniferous Nuculse. The external ornament in the 

 Nucidange of Carboniferous age is always regularly linear, a character which does 

 not always obtain in the Nuculse of the same period. I prefer, therefore, to retain 

 the two genera, and consider that the absence of a single character (non- 

 sinuated pallial sinus), not enough to warrant the separation of shells from the 

 genus Nuculana which possess all the other special characters of that genus. 



Of the six species described as Nucula by Phillips, one, N. cuneata, cannot any 



