THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 523 



Ctenodonta triangularis, sp. nov. (PL TIL, figs. 10, 11.) 



Specific Characters. — Shell compressed, triangular. The anterior end deep, the 

 posterior narrowed and slightly produced. The anterior border descends downwards 

 and backwards almost straight ; the inferior border is bluntly curved at first, and almost 

 straight in the posterior two-thirds. Posterior border very short, truncate. Hinge 

 line almost straight, the longest diameter of the shell. The umbones are small, 

 pointed, and inconspicuous, subcentral. 



Interior. — The. anterior part of the hinge plate only has been seen, and consists of 

 a number of small tubercular teeth. 



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

 growth. 



Dimensions. — Fig. 10, PL III., measures : antero-posteriorly, 6 "5 mm. ; dorso- 

 ventrally, 4 mm. 



Locality. — Llandeilo series of Ardmillan. 



Observations. — Two specimens of the species have been obtained. The specimen 

 on which my description is based is a right valve. Its shape and contour are so peculiar 

 that it cannot be referred to any known species. 



The whole of the hinge has not been seen, but for the present I refer it to 

 Ctenodonta. 



It has occurred to me that the Posidonomya venusta figured by Portlock may be 

 an example of the shell 1 have j ust described. 



Ctenodonta aff. transversa, Portlock, sp., 1843. (PL III., figs. 12-14.) 

 Area transversa, Portlock, 1843, Geoh Rep. Londonderry, p. 428, pi. xxxiv. figs. 1-4. 



Specific Characters. — Shell almost equivalve, transversely oval, ends somewhat 

 truncate, the posterior broader than the aDterior, very moderately gibbose. The 

 inferior margin broadly convex, the hinge line arched. Umbones small, very little 

 raised, subcentral. No lunule or escutcheon. 



Interior. — Normal. 



Exterior. — Surface almost smooth, but the microscope shows very fine concentric 

 lines of growth. 



Dimensions. — Fig. 13, PL III., a left valve, measures: antero-posteriorly, 15 mm. ; 

 dorso-ventrally, 8 mm. 



Localities. — Scotland : Llandeilo bed, Balclatchie, Craighead ; . Middle Bala of 

 Shalloch Mill and Whitehouse Bay ; Lower Llandovery, Mulloch Hill ; Middle 

 Llandovery, Woodland Point ; Upper Llandovery of Penkhill. 



Observations. — The Scotch specimens are very much smaller than Portlock's types, 

 which are all internal casts, and for this reason it is difficult to be certain that I 



