152 INVEKTEBHATA. 



striae. A set of well-defined regular concentric striae crosses the 

 ribs and interspaces, producing a distinct scalariform structure. 



*The cast is smooth or faintly radiated, with a posterior groove 

 left by the muscle-shelf. 



Measurements. Cardino-pallial diameter, 19 mm. ; antero-pos- 

 terior diameter, 21 mm. ; thickness (through both valves), 14 mm. 



Affinities and differences. This shell is so very nearly equi- 

 lateral that it might be referred to the genus Pectunculus, but it 

 has the projecting shelly muscular shelf for the insertion of the 

 posterior adductor muscle as in Cuculhea. Its globose, nearly 

 equilateral form, sharp ribs with few striations, and the regular 

 cross-barring of the interspaces well distinguish this species. 



From C. rotundata, Roemer, it is distinguished by its larger 

 size, better-defined ribs, and larger more rounded umbo. 



I have little doubt that the original home of this shell was in 

 Neocomian waters, though now it is only known as a derived fossil 

 in newer Neocomian deposits. 



Localities. In the Black Grit rock, derived blocks, Upware ; 

 also in the Herrimere Rock. 



CUCULL^EA DONNINGTONENSIS, sp. 11OV. 



(Plate VIIL, fig. 9, a, 6.) 



Shell globose-quadrate, inequilateral, radiately ribbed. Umbones 

 prominent, rounded, oblique. Anterior side rounded ; posterior 

 obliquely truncated. Hinge line short ; area narrow, marked with 

 angular cartilage grooves. Ribs about 30, equal but becoming 

 faint towards the posterior side ; they are angular in section, mo- 

 derately prominent, and covered with strong secondary striae, 

 which also occupy the interspaces. These are crossed by close-set 

 growth striae. 



Measurements (type from Upware). Cardino-pallial diameter, 

 21 mm. ; antero-posterior diameter, 25 mm.; hinge-line, 12 mm. ; 

 thickness (through both valves), about 20 mm. 



A ribbed species more oblique than the last, and with the ribs 

 more triangular in section and covered, together with the inter- 

 spaces, with numerous and well-marked striee, which are crossed 

 by close-set lines of growth ; also the umbo is loss prominent. 



