220 



Of the six species of Corbula described by Mr. Gabb from the 

 Californian Cretaceous, this little shell comes nearest to C. parilis* but 

 it is much more diminutive in size, more inequilateral, and shows no 

 traces of radiating strise between the concentric ribs. 



PERIPLOMA CUSPIDATUM. (N. Sp.) 

 Plate 29, figs. 4, 4a and 4/>. 



Shell compressed but perhaps abnormally flattened; apparently 

 thin and lenticular, nearly equivalve, but very inequilateral : anterior 

 end subcircular in some specimens, more narrowly rounded in others, 

 usuaHy gibbous and somewhat produced at or near the termination 

 of the base in front ; posterior end shorter than the anterior, abruptly 

 cuspidate, either rounding more or less regularly upwards and outwards 

 from below, and forming an angular or subangular junction with the 

 hinge line above, or contracting suddenly and concavely above into a 

 short, narrow and upturned beak, of which the hinge line forms the 

 upper boundary. Ventral margin broadly rounded, but sometimes 

 gibbous or subangular in front : superior border convexly arched 

 in front and concave behind. Beaks broad, low, depressed, recurved 

 and placed considerably behind the middle. Posteriorly the beaks 

 are bounded by a single, short, oblique and narrowly linear groove, 

 which indicates the probable existence of a corresponding thin and 

 laminar rib on the inner surface of the posterior umbonal slope in 

 each valve. 



Surface apparently smooth or marked only by a few faint concentric 

 striations. 



Length in the middle of the valves, twenty millimetres : maximum 

 height twenty-four mm. and a-half. 



North side of Maud Island : two specimens with both valves flattened 

 out and a single right valve. 



This shell is evidently congeneric with the Periploma suborbiculatum 

 described on page 138 of the present volume from a single specimen 

 collected by Mr. James Richardson in 1872 from the Upper Cretaceous 

 rocks of the ISTanaimo River, V. I. It may be only a variety of that 

 species, but appears to differ therefrom in its narrowly and shortly 

 beaked posterior extremity, less central beaks and in having only one 

 laminar ridge on the interior of each valve. 



The lateral outline of the valves of some specimens of this species 

 is singularly like that of the Meekia sella of Gabb,f but the present 



* Palaeontology of California. Vol. I., p. 150. pi. 29, figs. 239 and 239a, 

 t Palaeontology of California. Vol. 1, p. 191 , pi. 25, fig. 179. 



