THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 517 



Dimensions. — Fig. 11, PL IT., measures : antero-posteriorly, 30 mm. ; dorso-ventrally, 

 45 mm. ; elevation of valve, 3 mm. 



Locality. — Woodland Point, Middle Llandovery ; Star-fish beds, Upper Bala. 



Observations. — Fig. 11, PI. II., agrees well with the specimen figured by Salter 

 (op. supra cit.). Amongst the specimens is one which I conceive has been much 

 compressed laterally, so that it appears to be a very narrow but deep shell ; and there 

 is a specimen (PI. II., fig. 12) that I think may be a very senile form, though it is not 

 unlike P. ponderosus of Hall. 



Salter suggested that the Inoceramus trigonus of Portlock might be a form 

 of P. mytilimeris, but the hinge line is much too long for this species. 



Genus Nucula, Lamarck, 1799. 

 For synonomy, vide Hind, 1897, Mono. Brit. Garb. Lamell., p. 175. 



Generic Characters. — Shell equivalve, inequilateral, close all round, more or less 

 gibbose. Hinge angular, multidenticulate ; hinge teeth in two sets, separated by a small 

 fossa for the ligament, which is internal. Adductor muscle impressions well marked. 

 Pallial line entire. No lunule or escutcheon. 



Observations. — I have thought it well to refer two species to this genus, on the 

 general characters and shape, although I have not been able to verify the generic 

 character of the hinge plate. Conversely to what obtains in Carboniferous times, Nucula 

 is only represented in Ordovician rocks by two species, while Ctenodonta has assumed 

 many different forms. 



For a fuller discussion of the relations of Nucula, Nuculuna, and Ctenodonta, I 

 would refer to my monograph (op. supra cit.). 



Nucula levata, Hall, sp., 1847. (PI. III., figs. 29-31, 31«.) 



Nucula levata, M'Coy, 1855, Brit. Pal. Foss., p. 285, pi. iv. figs. 4, 5. 



„ ,, Hall, 1847, Pal. New York, vol. i. p. 150, pi. xxxiv. figs. la-4. 



Specific Characters. — Shell obliquely sub-rhomboidal, gibbose. The anterior end 

 is rounded, inferior margin slightly convex, posterior margin oblique and almost 

 straight, making a bluntly rounded angle with the lower margin, and a very obtuse 

 angle with the hinge line. Hinge line almost straight, elevated, short. Umbones 

 elevated, swollen, pointed, incurved, and contiguous, placed almost about the centre of 

 the hinge line. No lunule or escutcheon. Dorsal slope hollowed and depressed. 



Interior. — Not known. 



Exterior. — The surface is almost smooth. 



Dimensions. — Fig. 30, PL III., measures : antero-posteriorly, 14 mm. ; dorso- 

 ventrally, 11 mm. ; from side to side, 10 mm. 



Locality. — Scotland : Llandeilo beds, Balclatchie, Ardmillan. Wales : Dinas Bran. 



Observations. — This species was stated by Hall to vary considerably, and the 



