138 PERMIAN FOSSILS. 



and separated. Crural base perforated. Foramen situated at the point of the umbone, 

 and open inferiorly by the fissure.'^ 



Type Atri/pa pectinifera, J. de C. Sowerby. 



CleiotJiyris, it will be seen by the above diagnosis, differs in several respects 

 from Atrypa, the genus in which its type has hitherto been placed ; and from Athi/ris, 

 with which it is more closely allied, in having the dental plates immediately attached 

 to the inner surface of the large valve, and in being without a medio-longitudinal plate 

 in the opposite valve. Probably it also differs from Athyris in the valves being 

 generally furnished with variously-characterised projecting lamellae of growth (as in 

 Cleiothyris planosulcata, Phill., C. pectinifera, J. de C. Sow., and C. ewpansa, Phill.) ; in 

 the spirals being pectinated (vide PI. X, fig. 10) ; and in having a perforate crural 

 base^ (vide PI. X, figs. 8, 9). Looking at the figure which Mr. T. Davidson has 

 published of the spiral apophysary system of the small valve of a species apparently 

 belonging to Athyris,^ there appears to be a wide difference between this genus and 

 Cleiothyris in their respective spiral crura, which, in the latter, are similar to those of 

 Trigonotreta ; whereas, in the former, they appear to be conjoined for a considerable 

 extent, so as to resemble a long wire-like process, forked at its termination, near the 

 centre of the shell, each fork appearing as if it passed into, or became connected with, 

 the spiral to which it is directed. 



The projecting lamellae of growth are evidently the same as those characterising 

 certain Discinas. The minute punctures noticed in the diagnosis are displayed in 

 some of my specimens of Cleiothyris amhigua {Spirifer id., J. Sow.) Respecting the 

 use of the perforation in the crural base, I am unable to offer any opinion. Has it any 

 relation to the wire-like process above noticed ? 



1 am not aware of the occurrence of species of this genus in any other than 

 deposits belonging to the Carboniferous and Permian systems. 



Cleiothyris pectinifera, /. de C. Sowerby. Plate X, figs. 1-10. 



"Spheroidal" Terebbatula, /. de C. Sowerby. Trans. Geol. Soc, 2d series, vol. iii, 



p. 119, 1829. 



Atkypa pectinifera „ Min. Conch., vol. vii, p. 14, pi. 616, 



(aU the figures,) 1841. 



be used in such a sense : further, as a group of shells hitherto placed in Atrypa requires a name, I have 

 made free to adopt Professor Phillips's rather than propose one myself, a step which has one recommendation 

 in its favour, that of using up a synonym. 



^ The following is the diagnosis given by Professor Phillips : " Cardinal area obsolete, beak incurved 

 over a minute perforation, which is often obtect, or merely serves to receive the beak of the smaller valve." 

 (Paleeozoic Fossils, p. 55.) 



2 Cleiothyris amhigua {Spirifer id., J. Sow.) has a perforated crural base. 



3 Vide Bull, dela Soc. Geol. de France, 2°'=serie, t. v, pi. iii, fig. 26. 



