2S0 BRITISH SILURIAN BRiVCIIIOPODA. 



peculiarity ; in tliem the radii are numerous, and augment considerably in number 

 by bifurcation, as well as by the interpolation of additional smaller ribs between some of 

 the larger ones. Our English specimens occur likewise under the shape of internal 

 casts and external impressions, the shell itself having disappeared ; but these last appear to 

 agree Avith similar casts and impressions sent to me by M. de Volborth, from Pulkova, in 

 Russia. In the internal cast of the ventral valve the sloping area, triangular prominence 

 due to the large pseudo-deltidium, and deep median slit, which commences at the beak, 

 and extends to about half the length of the valve, are features easily recognisable. 



Position and Locality. Prof. M'Coy states that his specimens, or rather those found 

 by Prof. Sedgwick, occur in the Upper Bala or Lower Llandovery at Cefn Coedog, 

 Corwen, North Wales, and that the species is less commonly met with in the schists of 

 Cyrn-y-Brain, west of Wrexham, Denbighshire. The best examples I have seen were 

 found by Mr. Parrott in that portion of the west flank of the Berwyn Mountains which 

 lies contiguous to the little village called Cynwyd, about three miles from Corwen, in 

 Merionethshire. The upper beds, from which he obtained the best specimens, lie on 

 the same horizon as the Cefn Cocli beds, near Mynydd Pron P'rys, near Cefn Cerrig, 

 though six miles apart. 



Abroad it is very abundant, at Pulkova, Pavlosh, Tzarskocelo, Jumalasari, &c., in 

 the neighbourhood of St. Petersburgh, also in the sandstone of the River Vloia, 

 Esthonia (?), &c. 



Genus — Strophomena, Rafinesque. 

 See "General Introduction," p. lOf). 



As the etymology of the name (nrpot^oc, band, and unvn, crescent) indicates, the 

 generality of the species composing this genus are more or less semicircular, and widest 

 at their long straight hinge-line. Their valves are more or less concavo-convex, regularly 

 arched, geniculated, or depressed. It so happens, however, that the valve which is 

 convex in some species is concave in others, and vice versa. It differs also from Orthis in 

 having a closed fissure ; that is to say, the fissure in the ventral valve is always to a 

 greater or lesser extent arched over by a convex pseudo-deltidium, while the projecting 

 bifid cardinal process, which is also in some species partly covered by a narrow pseudo- 

 deltidium, fills up the entire cavity of the fissure that may not have been arched over by 

 the pseudo-deltidium of the opposite valve. A minute circular foramen exists also at 

 the extremity of the beak in the ventral valve of some species, but becomes generally 

 cicatrized at some period of the animal's existence. 



In the interior of the dorsal valve the cardinal process is bifid or bilobed. 



