352 DR WHEELTON HIND ON THE LAMELLIBRANCH AND 



Exterior. — The surface is almost smooth, but with the microscope fine concentric 

 lines of growth are visible. The valve is crossed by several radiating obscure flattened 

 ribs, apparent even in casts. 



Dimensions. — PI. II. fig. 60. Specimen No. Tl984 B , a right valve, measures : antero- 

 posteriorly, 19 mm; dorso-ventrally, 7 mm. 



Locality. — Dumbartonshire, Greenfoot Moulding Sand Quarry, near Gain Farm, 

 3 miles N. of Coatbridge. 



Observations. — The species differs from S. costellata, M'Coy, being much more 

 gibbose and comparatively more transverse and much narrower. The radiating ribs 

 are much less obvious. Three specimens are present in the collection. It is possible 

 that the species is a mutation of S. costellata, for there is no doubt that the two forms 

 are closely related, but the differences seem to me to be sufficiently strong to be 

 indicated by a new term. Apparently the genus is represented in the Coal Measures 

 of Illinois, for the name of the genus occurs twice in the list on page 126 of the Final 

 Report of U.S. Geological Survey of Nebraska, but no mention is made of any species 

 in the text. 



Genus, Prothyris, Meek, 1869. 



Prothyris, Meek, 1869, Proc. Acad. Nat. Sci. Philad., p. 172. 



„ Meek, 1871, Amer. Jourri. Conch., vol. vii. p. 5, pi. i. fig. 3. 



„ Meek, 1872, Fin. Rep. U.S. Geol. Surv. Nebraska, p. 223, pi. x. fig. 9, a, b. 



„ Hall, 1885, Geol. Surv. N. York, Pal., vol. v. pt. i., Lamellibr. ii., p. xl. 



„ Miller, 1889, N. Amer. Geol. and Pal., p. 504. 



„ Whidborne, Pal. Soc, 1890, Devonian Fauna, p. 86. 



Generic Characters. — Shell compressed, narrow, transversely elongate, with a small 

 ear-like process from the antero-posterior angle, separated from the shell by an angular 

 ridge, probably for the byssus. Dorsal slope compressed. 



Observations. — The genus is easily recognised by the peculiar-shaped process at its 

 anterior end. The genus was founded by Meek for a shell from the Coal Measures of 

 Nebraska. The original account (op. supra cit.) was very meagre, and unaccompanied 

 by any figures. However, in 1871 he gave figures, and, I presume, a more elaborate 

 description, and again in 1872 (op. supra cit.). In North America the genus was sub- 

 sequently found to occur as low as the Hamilton series, where it is represented by two 

 species. Two others are known from the Chemnung and one from the Waverley Sand- 

 stone. In England, the Rev. G. F. Whidborne has described three species from 

 the Marwood and Pilton beds of North Devon. 



The internal characters are not known. I think it probable that the ear-shaped 

 process contained the anterior abductor muscle, and the hollow separating it from the 

 rest of the shell was for a byssus. Fischer doubtfully places the genus in the Solemdae. 

 Apart from the anterior ear-like process, the shell has a strong resemblance to some 

 species of Sanguinolites. 



