142 MOLLUSCA FROM THE GREAT OOLITE. 



depressed; a narrow sulcation passes nearly perpendicularly from the uinbo to the 

 inferior border ; the sides of the valves have numerous closely arranged and nearly regular 

 but depressed plications; the plications towards the extremities are crossed by a few 

 radiating and rather irregular costae, which are most prominent upon the anterior side, 

 but upon each side the costae become indistinct which are nearest to the middle of the 

 shell. 



Lateral diameter, 13 lines; height, 9 lines. 



Geological position and locality. The specimen kindly forwarded to us by Dr. Murray 

 of Scarborough, is from the Grey Limestone of the Great Oolite. 



PHOLAS COSTELLATA. Tab. XIII, fig. 1 8. 



Testa parvd, ovatd, antice convexd, costettatd, postice attenuatd sub-lcevigatd ; umbonibus 

 magnis ante mediums, compressis ; valvis in media sulco obliquo ; costellis prominentibus 

 subacutis irregularibus ; laminis concentricis crebris depressis. 



Shell small, ovate ; anterior side convex, with radiating, irregular, subacute costae ; 

 posterior side nearly smooth ; the middle of the valves is depressed, with a groove which 

 passes obliquely downwards and backwards ; the umbones are placed anterior to the middle 

 of the valves, they are large and compressed ; the extremities of the valves are nearly 

 closed ; the concentric lamellae upon the sides of the shell are fine, and closely arranged, 

 nearly disappearing upon the posterior half of the valves. The calcareous crypt, which 

 contains the shell, is obtuse anteriorly. 



Compared with Pholas Oolitica the costae are more distinct and numerous upon the 

 anterior half of the valves ; the concentric lamellae are more numerous, closely arranged, 

 and much less conspicuous, so that they scarcely impress the radiating costse ; the mesial 

 sulcus is more oblique, and the general figure of the shell is less cylindrical ; the crypt is 

 less ovate, or more nearly pyriform, the anterior extremity being more obtuse. The 

 posterior extremity of the shell is somewhat embedded in the crypt, and is not sufficiently 

 exposed to enable us to give the dimensions with accuracy ; the costse upon the anterior 

 half are about 12 in number, and nearly straight ; the anterior extremity does not exhibit 

 any distinct apertures. 



Locality. Scarborough. 



CEEOMTA CONCENTRICA. Tab. XV, fig. 3, antea, p. 108. 

 GERVILLIA ACUTA. Tab. XIV, fig. 1, la, antea, p. 20. 

 Trigonia conjungens is probably a variety of T. angulata. 



NOTE. We are indebted to the liberality of Mr. J. Leckenby and Mr. W. Bean, of (Scarborough, for the 

 loan of the specimens above described from the Oolite of Yorkshire. 



