504 DR WHEELTON HIND ON 



being less transverse than the other. He figured the more transverse form. The 

 specimen fig. 21, PI. II., in the Gray Collection is, I take it, an example of the less 

 transverse form. 



Modiolopsis exasperatus, Phillips, sp., 1848. (PL II., figs. 13-15.) 



Mytilus exasperatus, Phillips, 1848, Mem. Geol. Surv. Gt. Brit., vol. ii., pt. i., p. 364, pi. xx. fig. 12. 



Specific Characters. — Shell triangularly ovate oblique. The anterior end small, 

 compressed, its margin elliptical. The inferior margin sinuous in front, broadly rounded 

 posteriorly. Posterior margin arched and extended, making an obtuse angle with the 

 hinge line. The latter straight, short. Umbones prominent, incurved and twisted 

 forwards, slightly raised, placed in the anterior third of the hinge line. The valve is 

 obliquely swollen from the umbo downwards and backwards, the gibbosity becoming 

 broader and more rounded as it approaches the postero -inferior angle. In front of the 

 oblique ridge the anterior slope is rapid, and marked by the byssal sinus. The posterior 

 slope is more gradually compressed. 



Interior. — Unknown. 



Exterior. — The surface is ornamented with numerous distinct, sharp, concentric, 

 raised, narrow ridges, separated by narrow sulci. These ridges are finer and crowded 

 on the anterior portion of the valve. 



Dimensions. — Fig. 15, PI. II., a left valve, measures; antero - posteriorly, 

 52 mm. ; dorso-ventrally, 23 mm. ; gibbosity of valves, 15 mm. 



Locality. — Scotland : Llandeilo beds, Balclatchie, Star-fish bed of Upper Bala. 

 Wales : Myddleton series, Llandeilo. 



Observations. — A single specimen occurs in the Gray Collection from the Llandeilo 

 series. I cannot accept the reference of this species to Mytilus, on account of the marked 

 anterior extremity and non-terminal umbones. It has a typical modioliform shape. 

 The concentric ornament easily distinguishes the species from all others of the genus of 

 Ordovician age. 



Modiolopsis subquadratus, sp. nov. (PI. II., figs. 16, 17.) 



Specific Characters. — Shell small, oblique, subquadrate, valve gently convex. The 

 anterior border oblique, almost straight. The inferior almost semicircular, the posterior 

 more gently convex. The hinge line straight. The umbones are small, tumid, incurved, 

 subcentral. The shell is obliquely swollen from the umbo towards the postero-inferior 

 angle, the dorsal slope much compressed. The anterior end compressed. 



Interior. — Not exposed. 



Exterior. — The surface is ornamented with microscopically fine regular lines of 

 growth, the whole surface having somewhat irregular rugae and sulci of growth. 



Dimensions. — Fig. 16, PI. II., a right valve, measures: antero-posteriorly, 14 mm.; 

 dorso-ventrally, 12 mm. 



Locality. — Upper Llandovery of Penkhill and Bargany Pond Burn. 



Observations. — The figure of a shell named Modiolopsis expansa, Portl., is given on 



