THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 505 



p. 213 of the 3rd edition of the Silurian System: Fossils, Table 36, fig. 2, which from 

 its shape, I should suggest, agrees much more closely with the species under description 

 than Portlock's shell. The latter is transverse, and not nearly so deep in the dorso- 

 ventral measurements. 



The species does not appear to be common, and all the specimens in the collection are 

 about the same size. 



Modiolopsis scotica, sp. nov. (PI. II., fig. 18.) 



Specific Characters. — Shell of medium size, oblique, triangularly ovate, moderately 

 oblique. Anterior end small, narrow, but distinct, subacute. The anterior margin 

 descends downwards and backwards sinuously. The inferior margin is broadly rounded ; 

 the posterior obliquely truncate, almost straight, making a well-marked obtuse angle 

 with the hinge line, which is straight and of moderate length. The umbones are narrow, 

 slightly raised, and small, placed far forwards, but not terminal. 



Proceeding obliquely downwards and backwards towards the middle of the inferior 

 border, but soon becoming obsolete, is a subangular ridge, in front of which the shell is 

 much compressed by a well-marked byssal groove. The valves are compressed into 

 the postero-superior angle. 



Interior. — The anterior adductor muscle scar is comparatively large and deep, and 

 rounded ; between it and the umbonal hollow are two small accessory pedal scars. 

 Posterior adductor scar shallow and oval, normal in position. 



Exterior. — The surface is ornamented with few distinct concentric deep lines of 

 growth, more numerous and closer together near the lower margin. Elsewhere the 

 surface is nearly smooth. 



.Dimensions. — Fig. 18, PI. II., measures : obliquely, from anterior surface to postero- 

 inferior angle, 50 mm. ; transversely, 19 mm. 



Locality. — The Star-fish bed, Upper Bala. 



Observations. — A shell with the external appearance of Modiola, but having the 

 muscle system of Modiolopsis, I have referred it to the latter genus. It differs from 

 M. pyrus, Salter, in being more oblique and apparently not so gibbose, or regularly 

 rotund. Several specimens occur in the Star- fish bed, but many of them are crushed flat. 

 The two uncrushed examples are right valves, but these are imperfect posteriorly. 



This species is accompanied by a peculiar variety which is much less triangular and 

 has a short hinge plate, just as in the genus Naiadites a triangular form, N. modiolaris, 

 is accompanied by a N. quadratus, which is less oblique and has a shorter hinge. I 

 propose to call the variety as follows : — 



Modiolopsis scotica, var. brevis. (PL II., figs. 19, 20.) 



Varietal Characters. — Shell subquadrate, U-shaped, very slightly oblique ; hinge- 

 line equals the average transverse diameter of the shell. 

 External Characters. — As in M. scotica. 



