312 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ft. in. 



c. Ferruginous concretions, the upper part of the group, over which, "I 1 

 at Black-Gang Chine, the streamlet runs, immediately above the > to 



Cascade J 1 6 



Sand, subfossiliferous, brown and yellow 5 



6. A second range of ferruginous concretions, abounding in the im- ft. in. 

 pressions and vacant moulds of shells. This, near Walpen High-Cliff, 



is the part most abundant in fossils, which are easily detached 1 



The intermediate sand between a and b itself fossiliferous 7 



a. Consolidated ferruginous sand-rock, irregular in thickness, and in 



some places divided into two layers, containing many fossils 5 



Fossils of No. 45. 



45 a. Corbula striatula. Sow. Gervillia solenoides, Defr. {Forbesiana ?, 



Cyprina angulata ?, Sow. d'Orb. §) 



Thetis minor +, Sow. Lingula truncata. Sow. 



*Trigonia alaeformis. Park. Rostellaria Robinaldina, ctOrb. 



caudata, Ag, t*Turritella Dupiniana ? 



*Avicula pectinata ? Holaster complanatus. 



Gervillia anceps, Desk. 



45 b. Fossil coniferous wood. t*Cardium Voltzii, (VOrb. 



Panopaea plicata, Sow. fCorbis corrugata {Sphara, Sow.). 



t Neocomiensis, d^Orb. fCyprina angulata. Sow, 



tPholadomya Martini. f rostrata, Sow. 



fCardium peregrinosum, d'Orb. flsocardia Neocomiensis, d'Orb. 



h- : 



, X After comparing, with the assistance of Mr. Sowerby, a large number of 

 specimens of Thetis with the original species figured in the * Mineral Conchology,' 

 the correct determination of the different forms appears difficult, in consequence 

 of some of the characters being variable. 



The true Thetis minor (M. C. t. 513. f. 6), belonging to the ferruginous band 

 , No. 45 of the Table, is also found at Shanklin, Parham Park, &c., and has 

 f not, I believe, been discovered in the Gault of England, although M. d'Orbigny 

 cites it as common to that formation in France and this country ; but his figure 

 does not well agree with our species. It is characterized by the globose form 

 and narrow ligulate palleal sinus, which distinguishes it from T. major (M. C. 

 t. 513. f. 2, 4) of the upper greensand (Devizes, &c.), which has a broader sinus, 

 and the shell is more compressed, and attains a larger size. The species from 

 Blackdown, originally named Corbula Icevigata (M. C. t. 209. f. 1,2) by Mr. 

 Sowerby, and subsequently (Index) referred to T. minor, is less gibbose than that 

 species, and approaches therefore in this character to the specimens of Thetis 

 j; obtained from the " Crackers " group of the lower greensand ; the ligulate sinus, 

 £ however, and the palleal impression are similar to T. minor. M. d'Orbigny con- 

 , siders this a distinct species, and, under the name of Thetis leevigata, cites it as from 

 the upper portion (Terrain Aptien) of the lower greensand, and identifies it with 

 ^' those from the Crackers group in the Isle of Wight. The specimens figured in the 

 ? Pal. Franc, t. 387. f. 1, 2, do not appear to have well exhibited the sinus, palleal 

 impression, and the regularly radiating puncta, as D'Orbigny describes, the latter 

 being found only on the anal portion of the shell. The T. minor (M. C. t. 513. f. 5) 

 from Pinhay, near Lyme, is a gibbose species of the general form of the Isle of 

 V Wight specimens, but differs from them in the sinus gradually widening and being 

 • very broad at the base, as well as the palleal impression having a different ar- 

 rangement, agreeing in this respect with the figm-e of d'Orb. (P. F. t. 387. f. 4, 5). 

 If therefore the specimens from the " Crackers " (Isle of Wight) and Black- 

 ,^_down are the same, we must retain for them the name of Thetis Icevigata, using 

 "^^ T. minor for the more gibbose forms, as before observed. Roemer has united 

 ''-'■ T. minor and major under the name of T. Sowerbii. 



_, All the species are mai'ked by a regularly radiating series of minute raised 

 ""- puncta, which, when worn away, leave only small dots on the surface of the 

 shell, sometimes difficult to observe. — M. 



§ These shells are certainly nearer in form to G. solenoides than to G. Forbesiana, 

 which occurs lower in the series. M. d'Orbigny considers the species as distinct. — M. 



