ETHERIDGE DEVONIAN ROCKS AND FOSSILS. 649 



and Pilton. The typical genera, not known in higher beds, such as 

 Calceola, Merista, Pentameriis, Rensseleria, Retzia, Davidsonia, Stnn- 

 gocepliaJus, and Uncites (all peculiarly Devonian), appear to have 

 died out prior to the deposition of the Carboniferous series, during 

 those changes of conditions which are evidenced by the physical 

 structure of the Devonian sedimentary rocks, where through Xorth 

 Devon we recognize at the base great accumulations of red sand- 

 stones, succeeded by the fine and coarse gritty slates and impure 

 limestones of the Lynton or Lower Devonian group, and another 

 great and succeeding deposit of red sandstones and grits almost void 

 of organic remains (the Hangman Grits), and a second series (the 

 Ilfracombe group) of fine-grained fissile slaty deposits with asso- 

 ciated organic limestones, rich in corals and Brachiopoda, difi'ering 

 in the two British areas in the amount of limestone, but in nothing 

 as regards life-contents. This ^Middle-Devonian period was one 

 of continued depression, which must have gone on continuously, 

 as is proved by the position of the thick, glossy, fine-grained, un- 

 fossiliferous slates of Lee, Mortehoe, and Rockham, that overhe the 

 Ilfracombe fossiiiferous series (fcc, which sequence, with identical 

 conditions, can be detected in the South Devon area. These un- 

 fossiliferous Morte Slates were probably again brought within the 

 influence of shallow water ; and on them rest the now higher Upper 

 Old Eed Sandstone beds of Pickwell Down, and then succeeds the 

 upper marine Devonian series of Baggy, Marwood, and Pilton &:c., 

 with their crowds of typical Upper-Devonian species*. 



Singularly scarce are the remains of LamellibrcnicJis in this group 

 of deposits ; and the 12 genera and 17 species known are La- 

 minarian and deep-water forms — Avicula, Pterinea, Ctenodonta, 

 Corhida, the dwarfed Conocardia, Cucidlcea, &c. ; and indisputably 

 the marked assemblage of peculiar species of corals and Brachiopoda 

 accompanying them indicate the same bathymetrical conditions. 

 Two species only of the Lamellibranchiata, viz. Aviculopecten pli- 

 catics and Cardium cdifonne lived on to Carboniferous times : and 

 2 British species only are as yet known to occur in the three 

 European areas, as compared with the known South-Devon forms ; 

 and they are Conocardium cdifonne and C. mimix. 



Gasteropoda. — Similar facts are obtained by an examination of 

 the Gasteropoda of this Middle zone : 11 genera occur, many of them 

 being represented by only one or two species, such as Nerita, ScoJio- 

 stoma, Trochus, Turbo, and Vermetus ; 36 species are distributed 

 amongst the 11 genera, only four of which, or proportionally less 

 than half as many as the LamelUbranchs, lived on to the Carboniferous 

 Limestone ; they are, Acrocidia vetusta, Euomphahis Jcevis, Macro- 

 cheihis imhricatus, and Murchisonia spinosa ; whereas we know of 15 

 European species being common to our Devonian series of North and 

 South Devon. They are enumerated in Table Y. 



* Consult the able paper by J. W. Salter, Esq., on the Upper Old Red 

 Sandstone and Devonian rocks, Quart. Journ. Geol. Soc. vol. xix. for a com- 

 plete description of these beds and their fossil contents, as well as of those of 

 the Pembroke area. 



