494 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 20, 



Professor Huxley is right in referring this fossil to the genus Pter- 

 aspis — than which Ganoid Fish, abnormal though it be, there is no 

 more characteristic fossil of the Lower Cornstones of the Old Red 

 Sandstone. The Lower Devonian is therefore the equivalent of the 

 Lower Old Red Sandstone. 



There remain only the " Tilestones," or " Ledbury Shales," which 

 contain a different fauna from all these, but unequivocally (as I 

 believe) Lowest Devonian. But as these should form the subject 

 of a separate memoir, I will not trench upon that ground at present. 



§ 8. Appendix. 

 Curtonotus, gen. nov. 



It is necessary here to give the characters of a genus of Bivalves, 

 often quoted in the preceding pages, and which has long been known 

 in our lists of fossils without having been sufficiently illustrated. 



I believe I proposed the name in the joint memoir by Mr. Jukes 

 and myself in 1855. The genus is intended to include a number of oval 

 thick-shelled bivalves related to Myophoria and Aceinus, but distinct 

 at a glance by the general rounded form, and technically by the 

 simple, not divided, central teeth of the hinge. 



The genus is characteristic of the Pilton group throughout, occur- 

 ring at its extreme attenuated end, or immediately above it, in Pem- 

 brokeshire, and abundant throughout the Coomhola grits of South 

 Ireland, while it occurs, though rarely, in the Barnstaple slates. 



Some six or eight species are recognizable; some of which are 

 mentioned in the Explanations of Sheets 197, 198 of the Irish Sur- 

 vey, pp. 10 et seq. ; and one is figured in Professor Jukes's ' Manual,' 

 2nd edit. p. 508, fig. 14/. 



The following names are used in the Explanatory Sheets for the 

 Irish Survey, Sheets 197, 198 : — 



Curtonotus elegans, Salter, in Jukes's Manual, as above quoted. 



C. rotundatus, Salter, Expl. I. c. p. 11, Co. Cork, Ardgroom. 



C. elongatus, Salter, Old Head of Kinsale, Dunworley Bay. 



These are, on an average, 1^- inch wide ; and there are some still 

 larger species in the Irish cabinets. 



Generic Characters. — In the typical species, C. elegans, from Angle 

 Bay, Pembrokeshire, the beaks are prominent and placed at the an- 

 terior fourth. In one of the species from Co. Cork they are low and 

 nearly halfway from the anterior end. In C. elongatus they over- 

 hang the anterior side. However they may be placed, beneath them 

 in each valve is a thickened hinge-plate with a single strong trian- 

 gular central tooth, smooth-edged, and not indented below as in 

 Schizodus. In the left valve this tooth lies behind the deep notch 

 for the corresponding and equally large tooth in the right valve, this 

 being in front of it ; and in the right valve only is there an obscure 

 tooth behind the central one. There are no remote cardinal teeth, 

 nor hinge-lamella for the support of a ligament, which it seems must 

 have been external. The anterior muscular scar is deep, the poste- 

 rior less excavated, and placed far inwards ; the pallial impression 

 is entire, at a considerable distance from the edge in most of the 



