70 Proceedings of the Royal Physical Society. 



proposed — Pileojjsis by Lamarck, Plcdyccras by Conrad, and 

 Acroctdia by Phillips for these shells, but as they do not ap- 

 pear to possess any essential difference from De Montfort's 

 genus, these become obsolete.* Should it become necessary 

 to employ a separate name for the Palaeozoic shells, Plcdyccras 

 should be selected. Several sections of the genus have been 

 proposed, but as none of the species referable to these are 

 met with in Scotland, they will not be here referred to. The 

 species of the Scotch Carboniferous limestone number four 

 or five. 



The peculiar aberrant order of the Solenoconchi^ or Proso- 

 cephala is represented in our Carboniferous rocks by one 

 genus Dentcdium. The species met with in Scotland are 

 computed by Mr J. Young at five,-(- Dcntalium iiigcTis (De 

 Koninck), D. priscum (Goldf.), D. inornatioiii (M'Coy), D. 

 Scoticum (Young), and D. Dcdriense (Young). To these must 

 be added a sixth, Dentalium orncdum (De Koninck), found by 

 Mr J. Henderson.j According to Mr Young's researches, 

 the genus is not met with in any of the purer beds of lime- 

 stone, but from the accompanying shales, which he concludes 

 to indicate their abode in seas of less depth than those in 

 which the limestones were deposited. In the present un- 

 satisfactory state of the genera comprising the Dentaliidse, it 

 is impossible to indicate any generic distribution of the 

 species beyond the broad one of Dentalium. According to 

 Dr Stoliczka, this term should be restricted to certain 

 species with longitudinal ribs,§ a section which did not 

 flourish largely during Carboniferous times. 



We next pass to the Order Cyclobranchiata, of which the 

 Carboniferous representatives are Chiton and Chitonellus. The 

 first notice of this interesting group is contained in a paper 

 by Messrs Young and Kirkby,ll in which are described 



* See a summary of this subject, Annals Nat. Hist., 1880, vi., p. 296. 



t " On the species of Dentalium found in the Carboniferous Strata of tlie 

 W. of Scothuul" (Proc. Nat. Hist. Soc, Glasgow, 1880, iv., pt. 1, p. 69). 



X Etheridge, Geol. Mag., 1878, v., p. 117. 



§ Cret. Gasterop. of India, p. 437. 



il " Provi.sional notice of a new Chiton, and a new species of Chitonellus, 

 from the Carboniferous Rocks of the W. of Scotland" (Trans. Geol. Soc, 

 Gla.sgow, 1865, ii., pt. 1 p. 13). 



