388 CARBONIFEROUS FOSSILS. 



in the cast,* and in the flatter valve the new one frequently 

 remains for a long distance much smaller than the others, 

 and even vanishes and reappears after an interval. Both striae 

 and furrows are crossed by numerous fine decussating lines. 



No traces of spines are visible in our few specimens, which 

 agree in all respects (except perhaps in having a somewhat 

 thicker shell) with British examples of the P. cora. It is very 

 interesting to find this species, which is so universally spread 

 over the warmer parts of the globe, from India to the Andes, 

 and which has been described from Siberia and Bear Island, 

 ranging into these extreme northern latitudes. 



LOCALITY. Top of Exmouth Island, in situ, in a reddish 

 limestone of a kind not found at Depot Point. N.B. The 

 bones of the Ichthyosauri were found in close proximity, on 

 the highest point of the Island (Belcher) . 



PRODUCTUS SEMIRETICULATUS (Martin), 

 var. FRIGIDUS. 



Plate XXXVL, fig. 13, 14. 



Five inches wide and nearly three long ; larger valve very 

 gibbous at the beak, strongly and widely bilobed; ears pro- 

 duced and very distinct ; concentric plaits less strong than the 

 others ; spines numerous ; a distinct row parallel to the hinge 

 margin, and another obliquely sloping from it. Under valve 

 at first flat, then bent strongly down at light angles. 



Notwithstanding the wide bilobation of this shell, we can- 

 not think it anything but a conspicuous form of the variable 

 P. semireticulatus . It has the same kind of coarse striation 

 and concentric decussating plaits, very well shown in the left 

 half of our figure 3, where the lower or dorsal valve is seen, 

 the rest being broken away. 



The form is certainly more transverse than usual, and the 

 ears very distinct. In these particulars it resembles P. ex- 



* This should always be borne in mind in describing fossil bracluo- 

 pods, viz. that a rib interposed between two others will appear in the 

 cast of the surface as a dividing furrow on a ridge. 



