6?5> 



REroKT— 1S63. 



* A difficulty attends the identification of young specimens of these rare species, no 

 series having yet been obtained. " C. exeavata, var.," in Mus. Cum. is exactly interme- 

 diate between the two. The young oi excavata has a large swelling umbo projecting beyond 

 the margin ; the umbo in "?var." has the margin spreading round it, as in onyx, ym., 

 and in consequence appears turned in the contrary direction. The umbihcus above the 

 deck exists in both forms ; but it is not an absolutely constant chai'acter, even iia aduuca, 



108 



