THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 515 



are crushed so that the umbones and oblique swelling become angular. I can find no 

 described form at all like the shells under description, and have given them the specific 

 name of distans, on account of the widely separated umbones. 



Genus Par area, Hall, 1885. 

 Pararca, Hall, 1885, Geol. Surv. New York, vol. v., pt. i., Lamell., ii., p. xxxvi. 



Generic Characters. — " Shell equivalve, inequilateral, transversely subelliptical or 

 rhomboidal in outline. Posterior end often subtruncate. Anterior end short and 

 rounded. Valves moderately convex. Cardinal line about half the length of the 

 valve, arching at the beaks. Umbonal slope often defined. Surface marked with more 

 or less slender radii with narrow interspaces and with fine concentric stria? of growth. 



" Hinge narrow and long, furnished with a series of minute crenulations. Ligament 

 apparently contained in a narrow groove along the cardinal border. Muscular impres- 

 sions and pallial line unknown." 



Observations. — I have quoted Hall's diagnosis of the genus, to which he assigns 

 six species from the Devonian rocks of North America, of which P. transversa most 

 closely resembles the Scotch specimen. There is some little difficulty in settling which 

 is the type ; after the name of the genus we find in brackets, " Types, P. transversa, 

 P. venusta, P. Sao, P. recta," but the description of P. precedens comes first, and 

 P. transversa second. 



Pararca tenuilineata, sp. nov. (PI. IV., fig. 22.) 



Specific Characters. — Shell small, transverse, slightly inequilateral, regularly 

 swollen. The borders are formed by a single curve of varying radius. The hinge 

 line gently arcuate. The umbones are subcentral, small, moderately raised. 



Interior. — Unknown. 



Exterior. — The surface is everywhere crossed by close, fine, radiating lines, visible 

 under the microscope. These lines are a little thicker where they terminate at the 

 margin of the valve. There are several concentric undulations of growth on the 

 surface of the valve. 



Dimensions. — Fig. 22, PI. IV., measures : antero-posteriorly, 10 mm. ; dorso- 

 ventrally, 6 mm. 



Locality. — Penkhill, Upper Llandovery. 



Observations. — This is a very distinct little shell, and I regret that only one 

 specimen, and that one far from perfect, is at my disposal. It is very closely allied 

 to P. transversa, Hall, but it has a close resemblance to C. inequistriata, Beushausen, 

 from the Upper Devonian (Abh. Kbnig. Preuss. Geol. Landesanstalt, Neue Folge, 

 Heft xvii. ; Die Lamell. des rheinischen Devon., p. 347, pi. xxxvi. fig. 11). 



This author shows the posterior side of the shell to be a little longer than the 

 anterior, and if this obtains in the Penkhill specimen the latter will be a left valve. 



