182 PERMIAN FOSSILS. 



from the umbones, and diverging towards the superior (ventral or inferior, nobis) 

 portion of the anterior (posterior, nobis) slope ; whole shell covered with irregular, 

 rearly 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 length 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 PI. XV, fig. 20) : perhaps it 

 is a distinct species, — a view somewhat supported by the form of a young individual 

 represented in PI. 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 Modiola Pallasi ; but incor- 

 rectly ; as the latter has no teeth. 



PleurojjJiorus costatus appears to have had an extensive geographical range. It is a 

 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 locahty " 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. 3'2. 



