LEPTODESMA. 123 



Inferior margin gently and evenly convex. Postero-inferior corner broadly 

 rounded. Posterior margin gently concave. Hinge-line straight, and very nearly 

 equal to the greatest length of the shell. Contour of surface convex in the centre, 

 steeper in front, and slightly concave behind the umbo. Surface covered with 

 numerous, very irregular, prominent growth-lines. 



Right valve transverse, convex, subtriangular, oblique. Umbo smaller and 

 narrower than that of the left valve, and not reaching above the hinge-line. 

 Anterior wing convex, rather narrow, defined by an indistinct oblique line from 

 the apex. Posterior wing long, narrow, concave, undefined. Anterior margin 

 long, very oblique, slightly and broadly concave under the wing. Postero-inferior 

 margin convex. Posterior margin apparently incurved. Contour of surface steeply 

 convex across line from umbo to postero-inferior corner, being steeper behind than 

 in front. Surface apparently similar to that of the other valve. Hinge-line long, 

 bearing two very long, slight, horizontal lines, like teeth or ligamental grooves, 

 behind the umbo. Anterior muscle-scar large, deep, circular, situated in the 

 centre of the anterior wing, just in front of the umbo. 



Size. — Left valve : height 19 mm., length 29 mm., depth about 4 mm. A 

 right valve measures 13 mm. high, 25 mm. long, and about 3 mm. deep. 



Locality. — There are two left valves and one right valve from Pilton in the 

 Porter Collection. 



Remarks. — Mr. Porter's fossils are in a fair state of preservation for these beds, 

 and seem to be sufficient to define the species. Although the right and left valves 

 are on different slabs, their agreement is such as to leave no doubt that they belong 

 to the same species. 



Affinities. — It presents much similarity to Avicula crenato-lamellosa, Sandberger, 1 

 as given by Freeh, 2 but the present evidence points decidedly to its distinctness from 

 it. In our specimens the front wing is much larger, and the posterior side less con- 

 cave, and there are no signs of the existence of any radiations. A. pseudo-lsevis, 

 Tollman 3 , and OEhlert 4 , is regarded by Freeh as a synonym of Sandberger's shell. 

 It appears to be distinguished from our specimens by its scalloped concentric 

 striae, by its greater height and more elongate hind wing, and by other points. 



PteroniteUa venusta, Billings, 5 of the Upper Silurian seems longer and narrower, 

 with a smaller umbo and a smaller front wing. 



The Ludlow shell which Sowerby identifies doubtfully with Pterinea retroflexa, 



1 1853, Sandberger, ' Verst. Ehein. Nassau,' p. 288, pi. xxix, fig. 16. 



2 1891, Freeh, ' Abhandl. Geol. Specialk. Preuss.,' Band ix, pt. 3, p. 49, pi. iv, figs. 5 — 5 c, and 

 13—13 I. 



3 1885, Tollman, ' Ver. n. Vereins Eheinl.,' vol. 42, p. 199, pi. v, figs. 2, 2 a. 



4 1882, (Ehlert, ' Mem. Soc. Geol. Fr.,' ser. 3, vol. ii, p. 23, pi. Hi, figs. 5, 5 a. 



5 1874, Billings, ' Pala;oz. Foss. Canada,' vol. ii, pt. 1, p. 142, pi. ix, figs. 5—5 b. 



