182 PERMIAN FOSSILS. 
from the umbones, and diverging towards the superior (ventral or inferior, xo0é7s) 
portion of the anterior (posterior, 2047s) slope; whole shell covered with irregular, 
nearly obsolete, concentric wrinkles. Length five sixteenths of an inch; breadth 
upwards of half an inch.’ (Brown.) 
Pleurophorus costatus is a remarkably inequilateral species, the umbones being close 
to its anterior end: its dorsal and ventral margins are nearly parallel to each other ; 
and the lateral terminations are obtusely rounded: it has generally three or four, 
occasionally six, rather sharp ribs running from the umbones to the posterio-ventral 
margin ; and is furnished with a long corselet extending nearly the entire Jength of the 
back, and a well-defined lunette. The cartilage-fulcra are short, extending very little 
behind the umbonal points. The ridge posteriorly bounding the anterior adductor 
muscular impression, in each valve, is rather prominent, and marked, at its junction 
with the dental plate, with a small impression, evidently due to one of the visceral or 
pedal muscles. The dental system is represented in Pl. XV, figs. 16, 17; and the 
impressions of the adductor muscles and pallial line, in fig. 15, of the same plate. My 
largest specimen is an inch in width. 
A variety occurs at Byers’s Quarry, with the valves more tumid, the ribs decidedly 
less distinct, and the marginal outline more rounded (vide Pl. XV, fig. 20): perhaps it 
is a distinct species,—a view somewhat supported by the form of a young individual 
represented in Pl. XV, figs. 18, 19; since small specimens of the normal kind possess 
the same parallelism of the ventral and dorsal margins as those full grown. Should 
the so-called variety be hereafter found to constitute a separate species, it is proposed 
to name it Pleurophorus ovatus. 
Cypricardia striato-lamellosa, De Koninck, which possesses only two radiating ribs, 
appears to be a closely allied species: the same may be suggested of the so-called 
Nucula cuneata of Phillips, which differs, however, in being much narrower in front ; 
though some specimens of the present species have a tendency to this-form. Dr. 
Geinitz identifies the present species with De Verneuil’s M/odiola Pallas: ; but incor- 
rectly ; as the latter has no teeth. 
Pleurophorus costatus appears to have had an extensive geographical range. It isa 
common species at Byers’s Quarry, Suter-point, and another locality or two on the 
coast between Whitburn and Marsden rock: the beds containing it are supposed to 
be the highest of the Permian series. in Durham, and probably equivalent to the 
German Rauchwacke. Specimens, apparently dwarfed, as they rarely attain half the 
size of those found in Durham, occur in the Permian Marls at Newtown, near 
Manchester. A specimen bearing the locality “Stubbs Hill, near Doncaster,” is in 
the collection of the London Geological Society. It likewise occurs, not uncommonly, 
in the Shell-limestone of Humbleton Quarry, Tunstall Hill, and Silksworth ; also, but 
1 Transactions of the Manchester Geological Society, vol. i. p. 32. 
