62? 



REPORT 1863. 



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

 series having 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 beyond 

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

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

 deck exists in both forms ; but it is not an absolutely constant character, e?eu in aduncct. 



108 



