THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 525 



characteristic traits of the species are the central umbones and the angular ridge. I am 

 in doubt as to the correctness of referring the figure given by Murchison (op. cit. supra) 

 to Portlock's species, because the drawing shows neither of these characters. 



Ctenodonta eastnori, Portlock, sp., 1843. (PI. III., figs. 20-22.) 



Area eastnori, Portlock, 1843, Geol. Rep. Londonderry, p. 427, pi. xxxiv. fig. 3. 

 Ctenodonta eastnori, Murchison, 1859, Siluria, p. 233, pi. xi. fig. 9. 



Specific Characters. Shell gibbose. transversal, oval, compressed in front and 

 behind ; umbones subcentral, pointed, incurved. Posterior end slightly deeper than the 

 anterior; junction of anterior and posterior margins with the hinge line almost a right 

 angle. Hinge line straight, inferior margin slightly convex ; anterior almost straight, 

 passing downwards and forwards. Postero-inferior angle bluntly rounded ; posterior 

 margin straight, oblique. 



Interior. — Hinge line multidenticulate, no fossa separating the anterior and 

 posterior rows of teeth. Adductor scars small and not well marked. 



Exterior. — Surface nearly smooth. 



Dimensions. — Fig. 22, PI. III., a large specimen, measures: antero-posteriorly, 

 25 mm. ; dorso-ventrally, 10 mm. 



Locality. — Scotland : Llandeilo beds, Craigland, Ardmillan, Balclatchie ; Star- fish 

 bed, Upper Bala. Ireland : Desertcreat, Co. Tyrone. 



Observations. — This species seems to be rare in the Gray Collection. One specimen 

 (fig. 22, PI. III.) is much larger than the Irish specimens, which are casts of the interior, 

 and fortunately show the hinge plate. I have no hesitation in referring this species to 

 Ctenodonta. 



C. eastnori has been obtained from the Upper Llandovery of Malvern. 



Ctenodonta semitruncatus, Portlock, sp. (PI. III., figs. 23-25.) 

 Pectunculus semitruncatus, Portlock, 1843, Geol. Rep. Londonderry, p. 429, pi. xxxiv. fig. 7. 



Specific Characters. — Shell below medium size, equilateral, suborbicular, moderately 

 gibbose. The anterior end compressed, its margin rounded. The inferior margin 

 broadly convex, meeting the posterior border at a blunted angle. The posterior border 

 is obliquely truncate, almost straight. The postero-superior angle obtuse. The hinge 

 line only slightly arched. Umbones almost central, small, swollen, pointed and 

 incurved, slightly raised above the hinge line. Dorsal slope rapidly compressed, so that 

 there is an almost angular ridge from the umbo to the postero-inferior angle. 



Interior. — The adductor muscle scars are small and normal in position. Hinge plate 

 normal. Pallial line entire. 



Exterior. — The surface is ornamented with numerous fine, raised, concentric lines, 

 often interrupted by deeper, more irregular sulci of growth, and thus apparently grouped 

 into bands, especially towards the lower margin. 



