THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 507 



Observations. — Two specimens moderately well preserved are found in the 

 collection. One seems to show two small posterior lateral teeth ; the other, which is a 

 cast of the interior, is not so definite on this point. The blunt anterior and the 

 obsolete umbones are well marked. The ornament is markedly modioliform in 

 character. 



Genus Modiolodon, Ulrich, 1893. 



Modiolopsis, 1890. Amer. Geol., vol. v. p. 276. 



Modiolodon, Ulrich, 1897, Fin. Rep. Geol. and Nat. Hist. Surv. Minnesota, vol. iii. p. 521. 

 ,, Ulrich, 1893, Rep. Geol. Surv. Ohio, vol. vii. p. 652. 



Generic Characters. — Ovate shells of the same general type as Modiolopsis 

 and Modiomorpha, but having from one to three oblique cardinal teeth in each 

 valve. 



Observations. — The genus was established by Ulrich, who gave the brief generic 

 description noted above (op. supra cit.). I have referred several species to the genus on 

 external characters only, for in no case have I been fortunate enough to examine the 

 hinge. Ulrich points out that the hinge of Modiolodon is much like his other genus 

 Ischyrodonta, but he thinks the latter approaches the hingeless Modiolopsis in general 

 characters. 



Modiolodon subovaUs, Ulrich, 1893. (PI. II., figs. 30, 31.) 

 Modiolodon subovalis, Ulrich, 1893, Rep. Geol. Surv. Ohio, vol. vii. p. 635, pi. li. figs. 11, 13. 



Specific Characters. — Shell of moderate size, compressed, inequilateral, expanded 

 posteriorly, subovate, slightly oblique. The anterior end is short, the narrowest part 

 of the valve, its margin rounded. The inferior border gently convex. The posterior 

 obliquely subtruncate. The hinge line arcuate. The umbones are small, compressed, 

 and placed far forwards but not terminal. There is a tendency to angulation along a 

 line from the umbone to the postero-inferior angle, posterior to which the dorsal slope 

 is rapidly compressed. 



Interior. — The hinge has not been observed in the Scotch examples. The anterior 

 adductor scar is well marked and is deep, circular, and close to the antero-superior 

 angle ; the posterior, large and conspicuous, is seen in the hollow of the dorsal slope 

 not far from the postero-superior angle. Pallial line entire. 



Exterior. — The surface is adorned with close, fine concentric lines of growth, 

 separated into groups by distant linear grooves. 



Dimensions. — Fig. 30, PL II., a medium-size example, measures : antero- 

 posterior^, 30 mm. ; dorso-ventrally, 20 mm. 



Localities. — Middle Llandovery of Woodland Point and Newlands. 



Observations. — I have referred five specimens to this species rather than to Mytilus 

 gradatus, Salter, which should, I think, more correctly be referred to Modiolodon. 

 Ulrich describes several species which are closely related, but differ in contour and 



