62? 



RiroRT— ieG3. 



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

 eeries haying yet been obtained. " C. excavata, var.," in Mus. Cum. is exactly interme- 

 diate between the two. The young of excavata has a large swelling umbo projecting b«!yond 

 the margin ; the umbo in " ? var." has the margin spreading round it, fts in onyx, jun., 

 and ia consequence appears turned in the contrary direction. The umbilicus above tlia 

 dtsck exists in both forms ; but it is not an abiulutely coustuut ciiiuatter, uvtiu iu aduiwot 



108 



