158 PERMIAN FOSSILS. 



From the imperfect state of the only specimen I have seen of this species, it is 

 impossible for me to draw up any other than a provisional description of it. The 

 specimen is represented of the natural size in PI. XIII, fig. 24 ; but unfortunately, 

 owing to its not being sufficiently cleared of the matrix, its marginal outline cannot be 

 correctly delineated or determined. 



It agrees with certain varieties oi Monotis speluncaria in its tubuliferous ribs : it also 

 bears some resemblance to Schlotheim's Ostracites spondyloides of the Muschelkalk. 



Mr. E, Charlesworth has kindly allowed me to represent a testiferous specimen 

 (vide PL XIII, fig. 25^), found in the same locality where the present species occurs, 

 and exhibiting the inner surface of the flat or lower valve. It closely resembles the 

 corresponding valve of Monotis speluncaria, except in being thinner : it is probably the 

 under valve of Monotis Garforthensis. 



The specimen represented in PI. XIII, fig. 24, belongs to the York Museum 

 (ticketed No. 8837), and was found at Garforth. 



Order Dimyaria, Lamarck. 

 Diagnosis. — The valves attached to each other by two muscles laterallv situated. 



Family/ Mytilid^ {les Monies), Cuvier. 

 The common Mussel is the type of this group. 



Genus Mytilus, Linnaeus. 



Diagnosis. — " Shell equivalve, very inequilateral, subtriangular, more or less tumid, 

 surface covered with an epidermis ; beak terminal. Hinge without teeth, though often 

 more or less denticulated ; ligament linear, internal ; two unequal muscular impressions ; 

 pallial impression obscure, simple."^ 



This group is represented by the common Mussel. 



The shells next to be noticed do not seem to belong to the present genus, at least, as 

 it is generally defined, and judging of it by the characters of its typical species. Instead 

 of having the cartikge nearly concealed, situated in a fissure wider at the bottom than 

 at the top, and covered with the hinge-margins, they have it exposed, and situated in a 

 furrow wider at the top than at the bottom. They differ also from Mytilus, as above 

 described, in being inequivalved, and in having the anterior adductor muscle attached 

 to, and passing between, two oblique or horizontal plates in the point of the umbonal 



^ The portion on the left side of the line on the left side of the figure is restored. 

 2 Forbes and Hanley, British Mollusca, vol. ii, p. 168. 



