62? 



REPORT — 1863. 



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

 peries having yet been obtained. " C. excavata, var.," in Mus. Cum. is exactly intemie- 

 diato 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,]\m., 

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

 deck exists in both forms ; but it is not an absolutely constant character, even in aduaca. 



108 



