222 ARTHUR WILLEY. 
ever, important to note that in any case there is a distinct 
caudal fin extending to the posterior extremity, and Kirkaldy’s 
opinion that there is no caudal fin in Asymmetron is somewhat 
misleading. 
From the above description it is evident that A. caudatum 
comes very near to A. lucayanum, but differs definitely and 
distinctly from it in size, number of myotomes,! and in the 
possession of a prominent rounded rostral fin, marked off above 
and below by a constriction. 
i 
On tHE NEPIONIC SHELL OF THE Recent NavtiLus. 
(With figs. 5 and 6 on Plate 13.) 
While conversing recently with Mr. Charles Hedley, 
Conchologist to the Australian Museum, Sydney, I had 
occasion to refer to the very large size of the ovarian ova 
of Nautilus pompilius, apparently larger than the ova of 
any other known Cephalopod; while the fluidity of the yolk 
would lead to the presumption that the ova, when deposited, 
would be provided with a firm covering, whether horny, 
coriaceous, or calcareous. 
Mr. Hedley then drew my attention to two young shells of 
N. pompilius in one of the museum cases, in each of which, 
at the same region in both, there was a very distinct discon- 
tinuity in the formation of the shell by the animal, and added 
that this line of demarcation probably represented the limit 
reached by the shell at the time of hatching from the egg. As 
1 The average myotome formula for A. lucayanum as given by Andrews 
is 44—9—13 = 66, and the average length 13 mm. The maximum length, 
according to Andrews, is 16mm. out of a large number of specimens. 
Kirkaldy gives 19 mm. as the maximum length of specimens sent to Prof. 
Lankester by Prof. Agassiz. 
