THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 497 



Observations. — This shell has the umbones much less anterior than P. retrqflexa, 

 and the anterior end is much less sinuous. Only one specimen, a right valve, is in the 

 Gray Collection. 



Genus Leptodesma, Hall, 1883. 

 Leptodesma, Hall, 1883, Pal. New York, vol. v., pt. i., Lamell., i. p. 13. 



Generic Characters. — Shell aviculoid, oblique transverse, very inequilateral. 

 Anterior end nasute and acute, posterior expanded and compressed, alate, upper border 

 often produced into a linear spine. Hinge line with a slender lateral tooth, posterior 

 to the beak and parallel to the hinge line. Surface with concentric lines of growth and 

 smooth. 



Observations.— Leiopteria and Leptodesma are very closely related. Indeed, I 

 question the need for both genera. I have referred these species of Ordovician shells 

 to the genus, but the Carboniferous forms I placed in Leiopteria on account of the 

 shape of the anterior extremity. The genus is represented by a very large number of 

 species in North American Devonian rocks.. 



Leptodesma ardmillanensis, sp. nov. (PI. I., fig. 16.) 



Specific Characters. — Shell below medium size, somewhat rhomboidal, obliquely 

 gibbose, subalate. The anterior margin long, slightly sinuous ; inferior margin is 

 bluntly rounded, the posterior truncate, almost straight. The postero-superior angle a 

 rounded right angle. The hinge line straight, the longest antero-posterior diameter of 

 the shell. The umbo is gibbose, obliquely pointed, and placed about the junction of the 

 anterior and middle thirds of the hinge line. The greatest gibbosity of the valve is 

 subumbonal, the convexity gradually becoming less as the border of the shell is 

 approached. The postero-inferior angle is much compressed and slightly expanded, but 

 not produced. 



Interior. — There appears to be a single linear oblique tooth below the posterior part 

 of the hinge line. 



Exterior. — The surface is ornamented by few distant, linear, but well-impressed 

 concentric grooves. 



Dimensions. — Fig. 16, PI. I., measures: antero-posteriorly, 18 mm.; dorso- 

 ventrally, 17 mm. ; elevation of valve, 5 mm. 



Locality. — The Llandeilo series, Ardmillan. 



Observations. — The specimen on which this species is founded consists of two 

 portions, the shell and its impression. It agrees with a subsection of the genus 

 found in Devonian rocks of New York State, which have no posterior wing and no 

 projecting rostrum. L. umbonatum, Hall (Pal. N. York, Lamell., i. p. 198, pi. xc. 

 fig. 9), from the Chemung group, has a strong resemblance to the Scotch shell, but the 

 hinge line is comparatively longer. 



