THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 495 



The absence of radial ribs on the large posterior end will separate it at once from 

 P. asperula, M'Coy. The latter shell has a fine reticulate ornament. 



The specimen on which I have erected this species consists of a cast of the interior 

 showing the characteristic hinge plate of Pterinea, and a mould of the exterior from 

 which the surface ornament can be studied by a wax impression. Unfortunately the 

 posterior end is absent. 



Genus Opisthoptera, Meek, 1872. 



Opisthoptera, Meek, 1872, Proc. Acad. Nat. Sci. Phil., p. 319. 

 „ Meek, 1873, Ohio Pal, vol. i. p. 131, note. 



„ Ulrich, 1893, Rep. Geol. Surv. Ohio, vol. vii., Pal., p. 642. 



Generic Characters. — " Shell equivalve, usually triangular in outline, with the 

 beaks of moderate size, incurved and terminal, and the hinge line straight and very 

 long, in most cases forming a great posterior wing, length greater than the height. 

 Anterior side more or less abrupt. In the typical section, the greatest height is in 

 the anterior half, and the surface marked with numerous and frequently bifurcating 

 costae. In another group of species, provisionally regarded as congeneric, the 

 posterior part of the shell is the highest, and the radiating costse few and mostly 

 simple. Byssal opening, muscle scars, and pallial line as in Anomolodonta and 

 By ssony chia. Hinge with two small cardinal teeth in each valve, but so far as 

 is known, no posterior lateral tooth. External ligamental area usually narrow; no 

 internal ligament." 



Observations. — The above is Ulrich's description of the genus (op. supra cit.). It 

 is to the second group of species that I consider the Scotch specimen belongs which I 

 refer to the genus. The greatest dorso-ventral diameter is posterior ; the radiating ribs 

 are single and almost obsolete. The byssal orifice is much too small for By ssony chia, 

 and the shell is too transverse. 



Opisthoptera transversa, sp. nov. (PI. III., fig. 4.) 



Specific Characters. — Shell of medium size, very inequilateral. Anterior side 

 obsolete, moderately swollen, transversely subquadrate. The anterior margin abrupt, 

 adpressed, and excavated below the level of the edge of the umbonal fold. The inferior 

 margin prolonged, gently convex ; the posterior obliquely truncate and straight above, 

 rounded below. The hinge line straight, of moderate length, meeting the posterior 

 border at an obtuse angle. The umbones are compressed anteriorly, pointed, and 

 twisted forwards, raised and terminal. The dorsal slope is compressed and becomes 

 concave as it approaches the posterior margin. 



Interior. — Not exposed. 



Exterior. — The surface is covered with numerous subobsolete simple radiating 

 raised lines, more apparent towards the lower margin, and crossed here and there by 

 more or less well-marked concentric lines of growth. 



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