THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 485 



The antero- and postero-superior angles are compressed and flattened, well defined 

 from the umbonal gibbosity. 



Interior. — The hinge plate is flattened and edentulous. No trace of anterior 

 adductor muscles. Pallial line entire. 



Exterior. — The surface is ornamented by a fine, delicate retiform marking, the 

 meshes being diamond-shaped and transverse, and arranged concentrically. In the 

 centre of the valve are one or two very obscure but broad radiating ribs. The retiform 

 marking is continued to the angles of the valve. 



Dimensions. — Fig. 2, PI. I., measures : antero-posteriorly, 11 mm. ; dorso-ventrally, 

 18 mm. ; elevation of valve, 3 mm. 



Locality. — Lower Llandovery of Mulloch Hill. 



Observations. — The ornament of this shell is so striking that it is not likely to be 

 confounded with any other species. The specimen consists of a cast of the exterior and 

 a mould of the interior. Unfortunately, the former is incomplete at one angle and near 

 the umbo, and I am not able to say which valve it is. It is to be hoped that more 

 specimens of this interesting shell may be found. 



Protopecten crenulatus, sp. nov. (PL I., fig. 1.) 



Specific Characters. — Shell of moderate size, equilateral, subcircular, moderately 

 gibbose. The margin forms a single curve of varying intensity, and was probably 

 crenate in the adult. The antero- and postero-superior angles well marked, compressed, 

 and subalate. The hinge line of moderate length, straight. The umbones subcentral, 

 small, pointed, slightly elevated. 



Interior. — Not exposed. 



Exterior. — The surface is ornamented with concentric lines of growth, somewhat 

 rugose and irregular, and towards the margin markedly crenate, fine and close. 



Dimensions. — Fig. 1, PI. I., measures : antero-posteriorly, 25 mm.; dorso-ventrally, 

 28 mm. 



Locality. — Upper Llandovery of Penkhill. 



Observations. — The specimen on which I have erected this species is somewhat 

 obscure and incomplete. The crenulate markings of the anterior and lower margins, 

 and the approach of the two upper angles to ears, have induced me to regard the shell 

 as related to the Pectinidse. 



Genus Byssonychia, Ulrich, 1894. 



Byssonychia, Ulrich, 1894, Pal. Minnesota, Fin. Rep., vol. iii. p. 498. 

 „ „ 1893, Rep. Geol. Surv. Ohio, vol. vii. p. 629. 



Generic Characters. — Shell oblique, subovate and quadrate, very inequilateral, 

 gibbose, subalate, and compressed posteriorly. Umbones anterior, terminal, twisted 



