THE LAMELLIBRANCHS OF THE SILURIAN ROOKS OF GIRVAN. 521 



Nuculana curta, sp. nov. (PL IV., figs. 10-14, 14a.) 



Specific Characters.- — Shell small, triangular, ovate, gibbose, inequilateral. The 

 anterior end rounded, the posterior narrowed at the expense of its upper margin. 

 The anterior margin rounded, the inferior convex, the posterior narrow and blunt. 

 The hinge line arched. The umbones large, raised, incurved and pointed, placed at 

 the junction of the anterior and middle thirds of the hinge plate. The lunule small, 

 escutcheon large and wide. 



Interior. — The hinge plate appears to have a central cartilage pit between the 

 anterior and posterior rows of teeth. Muscle scars normal, but no impression within 

 the umbonal cavity. Pallial line entire. 



Exterior. — The surface is ornamented with close, regular, fine, linear, concentric 

 lines. 



Dimensions. — Fig. 11, PL IV., measures: antero-posteriorly, 10 mm.; dorso- 

 ventrally, 6 mm. 



Localities. — Middle Bala, Shalloch Mill ; Upper Bala, Thraive Glen and Star-fish 

 bed ; Lower Llandovery, Mulloch Hill. 



Observations. — A very well-marked little shell, which must be placed under 

 Nuculana on account of the large escutcheon and the arrangement of the hinge plate. 

 Not uncommon ; several examples show casts of the interior. This species is much less 

 transverse and pointed than N. imbricata, which occurs in the Llandeilo beds of 

 Ardmillan. 



Genus Ctenodonta, Salter, 1851. 



Ctenodonta, Salter, 1851, Rep. Brit. Assoc, for 1851 (1852), p. 64. 



Hind, 1898, Mono. Brit. Carb. Lamell. (Pal. Soc), p. 209. 



Generic Characters. — Shell more or less transverse, ovate. Hinge consists of 

 a long row of teeth, which become smaller as they approach the centre from 

 either extremity. There is no cartilage pit below the umbo separating the 

 hinge teeth into an anterior and posterior set. Pallial line entire. No lunule or 

 escutcheon. 



Observations. — On pp. 177 and 209, op. supra cit., I have discussed the synonymy 

 of this genus, which was established for a Canadian species by Salter, and it is 

 unnecessary to say more here. The genus was represented by many more species in 

 Ordovician than Carboniferous times. In the Devonian rocks of North America, Hall 

 shows that the number of species reached its maximum, and in Carboniferous times only 

 two species survived. The range in time is much less than either of its closely allied 

 genera Nucula and Nuculana. 



Most of the species which have been included in the Gray Collection have been 

 described previously, but one species I consider to be new, C. triangularis. 



