THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 493 



is very incomplete, much of the posterior side having been broken off, but the contour 

 of the valve can be recognised in the younger portion of the valve. The shell is a 

 striking one, its ornament so well marked that even fragments could be easily dis- 

 tinguished. I have referred the species to Posidonomya on external characters only. 



Pterinea, Goldfuss, 1836-40. 



Pterinea, Goldfuss, 1836-40, Petrefact. German., p. 134. 

 M'Coy, 1855, Brit. Pal. Foss., p. 258. 

 „ Hall, 1884, Geol. Survey New York, Pal., vol. v. pt. i., Lamell., L, p. xii. 



Generic Characters. — Shell unequi valve, inequilateral, posterior side alate, anterior 

 end nasute or auriculate. Ligament internal. Ligamental area longitudinally striated. 

 Cardinal teeth two or more. Lateral teeth linear, oblique. Posterior muscular 

 impression large, situated on the post-umbonal slope. Anterior muscular impression 

 small, situated within the rostral cavity. Test ornamented with rays. 



Observations. — I have adopted Hall's definition of the genus. M'Coy has 

 described several species from Silurian and Ordovician rocks (op. supra cit.). The 

 genus is represented in the Devonian rocks of North America by numerous examples, 

 but does not seem to have survived into Carboniferous time. All the Carboniferous 

 forms referred to the genus should be more correctly placed in Leiopteria and 

 Limatulina. 



Pterinea subfalcata, Conrad, sp., 1842. (PI. I., figs. 4, 5.) 



Avicula subfalcata, Conrad, 1842, Journ. Acad. Philadelphia, vol. viii., pi. xiii. fig. 4. 

 Pterinea subfalcata, M'Coy, 1855, Brit. Pal. Foss., p. 263, pi. i. fig v 3. 



Specific Characters. — Shell small, oblique, subquadrate. The body of the shell is 

 swollen and ovate. The anterior border is sinuous, the inferior broadly convex, the 

 posterior convex below, subfalcate above. The hinge line is straight, the longest diameter 

 of the valve. The umbo is small but tumid, subcentral. The anterior superior angle 

 compressed into a small lobe ; the posterior superior angle compressed and subalate. 



The interior appears to be normal. 



Exterior. — The surface is ornamented with fine thread-like radiating lines crossed 

 by close concentric ridges. 



Dimensions. — Fig. 1, PI. I., measures : antero-posteriorly, 12 mm. ; dorso-ventrally, 

 11 mm. 



Locality. — Craigens Quarry, Lower Llandovery ; Camregan Wood, Upper 

 Llandovery. 



Observations. — There are six examples of this little shell which I refer to 

 P. subfalcata, Conrad, on account of its oblique character and the short posterior ear. 



M'Coy figures a specimen from the schists of Howgill Fells, and states that the 

 species occurs at Benson Knot, Kendal, and near Pont-ar-y-llechan, Llandeilo. The 

 ornament is well seen on two specimens. I have only seen the left valve, the smooth 

 or right valve not occurring in the collection. 



