52 Proceedings of the Roycd Physical Society. 



found in our rocks of Carboniferous age. The most abundant 

 and very characteristic species, N. attemutta (Fleming, sp.), 

 like N'ucula gihhosa, possesses a well-defined cartilage pit im- 

 mediately under the beaks,* and like it, therefore, cannot be 

 absorbed in Ctenodonta (Salter). The name Peda, proposed by 

 Schumacher in 1817 has been in use for this particular group 

 of shells for many years ; but several authors, including 

 the late Dr Stoliczka-I* and Messrs Meek and Hayden, J 

 have shown that Nuculana (Link), published in 1807, has 

 precedence, and should be used in its place. The latter 

 authors have also pointed out the possibility of Yoldict 

 (Muller) being a Carboniferous genus. It is distinguished 

 from Nuculana by having a deeper pallial sinus, and less 

 prominent beaks. The Carboniferous shells, which especially 

 assume the outline of Yoldia, are N. clavata (M'Coy)§ and iY. 

 intermedia (Eth.).ll 



The extensive secondary genus Trigonia (Family Tri- 

 goniidce) is represented in Carboniferous rocks by Schizodus 

 (King), which many writers have confounded with Axinus 

 (Sowerby). The latter genus, as very justly pointed out 

 by Professor W. King,ir was typified by a shell from the 

 London clay (Eocene), but Sowerby united with it certain 

 Permian forms. The latter constitute King's ScJiizodus, 

 and are quite distinct from Sowerby's Axinus. Professor 

 M'Coy,** on the other hand, united Schizodus with Brown's 

 Myophoria ; but, although undoubtedly allied to this, I 

 think, with Meek and Hayden, f-j- that it is preferable to 

 retain the smooth and non-plicate Palaeozoic shells distinct 

 from the true Myophorice of the secondary rocks. 



In Scotland the Schizodi occur in the Carboniferous Lime- 

 stone, but in England their chief development appears to 

 have been in marine beds of the Coal Measures. A somewhat 

 similar habitat is assumed by them in the Calciferous Sand- 

 stone Series of Scotland, for along the south-east coast, where 



* Etheridge, Annals Nat. Hist., 1878, ii., t. 1, f. 6 and 7. 



+ Mem. Geol. Survey, India, iii., p. 319, J Pal. Up. Missouri, p. 60. 



§ Synop. Garb. Lime. Foss., Ireland, 1844, p. 11, f. 25. 



II Geol. Mag., 1873, x., t. 12, f. 3. 



II Permian Foss., England, 1851, p. 185. *"■ P>iit. Pal. Foss., p. 494. 



ft Pal. Up. Missouri, p. 58. 



