﻿78 MR. T. F. SIBLY OX THE i?AUXA.L SUCCESSIOX IX THE [Feb. I908, 



Chonetes. 



ChONETES Cf. HAEDEEXSIS, Phil. 



Specimens which are locally abundant in the shales of the 

 Cyatha.vonia'Subzone have a small, somewhat rectangular form, 

 with slight convexity, and are ornamented with very fine ribs, 

 increasing in number by forking. The hinge -line of an average 

 specimen measures 10 millimetres. 



Choxetes ceassisteia (M'Coy). 



Small, convex, alate shells, with coarse, simple ribs, occur rarely 

 in D3. These resemble very closely the Derbyshire specimen 

 described and figured by M'Coy in ' Brit. Pal. Foss.' 1855, p. 454 

 & pi. iii H, fig. 5, as Lejptcena (Chonetes) crassistria, but are smaller 

 than M'Coy's figured specimen. 



Choxetes cf. ceassisteia (M'Coy), Yaughan. 



I am unable to distinguish certain specimens, which are found 

 rarely in D., and D^, from the foim, described and figured by 

 Dr. A. Yaughan in Quart. Journ. Geol. Soc. vol. Ixi (1905) p. 294 

 & pi. xxvi, fig. 2, which abounds in the Cleist02)ora-Zone of the 

 South- Western Province. 



Ohoxetes compeessa, nom. no v. (PI. I, figs. 7 a & 7 h.) 



Description. — The form is semicircular, the hinge-line forming 

 the widest part of the shell. Both valves are almost completelj^ 

 fiat, the pedicle-valve being very slightly convex and the brachial 



valve very slightly concave. The 

 Fig. 6.— Median longitudmal sbell is thin. Each valve pos- 

 section of Chonetes compressa messes a well-developed area, that 

 (natural size). ^f ^^^ pedicle-valve being con- 



siderably the larger, and being 

 divided by a wide-angled delthy- 

 rium, which is partly closed by 

 /\ '^ a pseudo-deltidium. A row of 



spines project from the cardinal 

 edge of the pedicle-valve. 



The external surface of the 

 valves is covered with fine, thread- 

 like, slightly - flexuous, radiating 

 ribs, which increase in number 

 towards the margin by bifurcation 

 and by the intercalation of fresh 

 ribs. The ribs exhibit small 

 [^ = pediele-valve ; spine-apertures at intervals ; and 



6r = brachial valve.] the fairly - regular arrangement 



of these spine- apertures gives a 

 characteristic appearance to the shell. In the under layer of 



