102 ROYAL SOCIETY OF CANADA 



Platyceras radiatum (PI. V., figs. 5(7 and b). 

 Platyceras radiatum, Nat. Hist. Soc. N.B., Bull, xviii., p. litl, pi. i., figs. 5a and b. 



A small species of one whorl. Spire depressed, back rounded, but 

 with a faint carina indicated by a rib stronger than the others adjacent. 

 The shell has about a dozen tine, narrow, sharp ribs on the dorsum, 

 radiating from the umbo to the aperture ; and there are also faintly 

 marked and more distant transverse undulations, parallel to the aperture. 

 The aperture is a little campanulate in front and somewhat constricted 

 behind. 



Sculpture. — The surface is minutely granulate ; this granulation 

 extends over the ridges which traverse the dorsum, so that these ridges 

 are visible only by transverse illumination. 



Size. — Length along the whorl to the aperture, 4^ mm. ; width of 

 ai>erture 2^ mm. Infrequent. 



Distinct by its size and surface markings from any Cambrian species 

 known. By its radiate markings this species might be thought to be 

 connected with the genus Heliconopsis, Ulrich and Scotield, but the pro- 

 jecting umbo and dorsum seem to exclude it. 



Platyceras cymbula (PL V., figs. 6a and b.) 

 Platyceras cymbula, Nat. Hist. Soc. N.B. Bull., xviii., p. 191, pi. i., figs. 6a and b. 



A small species of one whorl ; spire depressed ; shell strongly car- 

 inate ; apex obtuse, turned slightly to the left and appressed nearly to the 

 anterior margin. On the dorsal half of the shell there are sharp thread- 

 like ridges radiating from the umbo to the aperture, where there are a 

 dozen or more ; these increase by intercalation ; obscure concentric swell- 

 ings at irregular intervals somewhat obscure the sharpness of the radiating 

 ridges. 



Sculpture. — As in the preceding species the surface is minutely granu- 

 lated. 



Size. — Length over all, 5 mm. ; length of the apertui'e, 4 raui. ; 

 width, 3^ mm. ; height, 2^ mm. Rare. 



This species is distinguished from the preceding by its sharper dorsum 

 and wider aperture 



Platyceras apertum of the Paradoxides Beds is distinguished from 

 the two preceding by the closeness and fineness of its radiating stria», and 

 from P. transrersum by the stronger undulations on the part of the shell 

 toward the aperture. 



' Lower Silurian Gasteropoda of Minnesota, Greol. Surv. Minnesota, vol. iii., pt. 

 ii., p. 826. 



