Vol. 67.] FROM THE PARADOXIDES BEDS OF COMLEY. 305 



ribbing, the interspaces being occupied to a smaller extent by inter- 

 polated costae. In this respect it comes nearer to Linnarssoirs type 

 of the species. 



Intermediate forms occur, connecting this variety with that which 

 I have named salopiensis. 



Locality and horizon. — Both sections on the Shoot-Eough 

 Eoad, in the Shoot-Eough-Eoad Flags : in the lower section 

 associated with var. sahpiensis and intermediate forms ; in the 

 upper section associated with Acrotreta aff. socialis. 



Btllingsella cobboldi, sj). nor. (PI. XXVI, figs. 20 a-23.) 



Shell semicircular in outline, biconvex, nearly twice as wide as 

 long, widest at the hinge-line; cardinal angles rectangular, or nearly 

 so. Valves ornamented with about ten or twelve principal strong- 

 rounded ribs. Additional ribs appear by interpolation at various 

 distances from the beaks, so that around the margin of each valve 

 some fifteen to eighteen ribs may generally be counted. Con- 

 centric lines of growth indent the ribs at close intervals. 



Pedicle-valve gently convex, with a very slight depression 

 along the middle. Area narrow, with small delthyrium ; beak in- 

 curved ; umbonal cavity small ; hinge-teeth small, divergent, 

 supported by small dental plates. Eibbing well marked in the 

 interior of the valve, but muscular impressions not shown. 



Brachial valve nearly flat. Area very narrow ; beak small ; 

 cardinal process simple ; muscular impressions not defined. 



Dimensions. — Length = 9 to 10 millimetres; width=16 to 

 17 mm. 



Locality and horizon. — The upper section on the Shoot- 

 Eough Eoad, Comley ; from the main mass of the Shoot-Eough-Eoad 

 Plags, about 4 feet below the Paradoxides-davidis band. 



Observations. — Five valves of this species, in the condition of 

 internal and external casts, have been obtained by Mr. Cobbold. 

 The British species which it most closely resembles is Orthis liicksii 

 Salter (Davidson) from the Menevian Beds of St. David's. 1 The 

 Shropshire form is, however, more transverse, the pedicle-valve less 

 elevated, the cardinal area lower, and the umbonal cavity smaller. 

 Its dimensions, too, are greater than those given by Davidson for 

 0. hicksii. The species also approaches a form found in the Para- 

 doxides-forclihammeri Zone of Sweden, which was originally 

 described by Linnarsson 2 as Orthis aff. hicksii and subsequently 

 designated by Dr. Walcott 3 Billing sella exporrecta var. rugosicostata. 



The generic position of this fossil is somewhat doubtful, as the 

 specimens do not exhibit the convex deltidium characteristic of the 

 genus Billingsella. 



1 See T.Davidson, ' Monogr. Brit. Foss. Brach.' (Pal. Soc. ) vol. iii (1869) 

 p. 230 & pi. xxxiii. figs. 13-16 ; and Suppl. Ond. vol. v (1882-84) p. 184. 



2 Op. jam cit. Bihang K. Svensk. Vetensk.-Akad. Handl. vol. iii (1875-76) 

 No. 12, p. 13 & pi. iii, figs. 22-23. 



3 Proc. U. S. Nat. Mus. No. 1395, vol. xxviii (1905) p. 236. 



