182 



II i:\URYj ON HYALODISCUS SUBTILIS. 



phery in point of measure or numerical value, to be about 

 the average or mean of these obtained on other more central 

 portions of the disc, the value of which I have estimated at 

 about 66 to 70 in •001", or much about the same as obtained 

 on Pleurosigma makrum, chiefly by white cloud illumi- 

 nation. 



Professor Bailey gives the relative proportion of the um- 

 bilicus of Hyalodiscus subtilis as being about one third the 

 diameter of the entire disc. I hence subjoin a series of 

 measures which may possess some share of utility, although 

 great variations are found to exist. 



Hyalodiscus subtilis and its Associates, &c.. 

 Upon Four Shells of each Slide. 



From whence, associated 

 with — 



California, Monterey, "I 

 Prof. Bailey . . J 



Ditto ditto 



Yarra Yarra, Campylo- 1 

 discus, Eupodiscns . J 



California, Aulacodls- 1 

 ctis oreganis, Trice- \ 

 tarium arcticum . J 



Kotzbue Sound, Uhuh- \ 

 donema Crozieri . I 



Ditto 



ditto 



Yarra Yarra 



California, Biddulpliia 

 Roperi, Aulacodis- 

 cus oreganis . 



Ratio of unibilicns to entire disc in parts of an inch 



r3ll 



1824 

 J 298 

 1778 

 rl94 

 1378 



J ISO 



UOO 



r2S0 



1637 

 r280 

 1700 

 r236 

 1466 



J2&i 

 1583 



311 

 1000 

 298 

 737 

 187 

 378 



233 

 53S 

 165 

 466 

 254 

 424 



Mean. 





entire shell I 306 "I 

 umbilicus ,1169/ 

 1 r! 2901 



li 867/ 

 n 212| 

 i 376/ 



1901! 

 400 jj 



225 1 I 



516/ 



216 1 



533/ 



2661 



479/ 



2371 



J 20/ 1 



1 521/ 



It hence appears that Hyalodiscus subtilis by no means 

 uniformly possesses an umbilicus, as formerly stated, at 

 about one third that of the entire disc, and so far as relative 

 proportion goes, can form no distinguishing feature from 

 Hyalod. Icevis, said to be about one half, see slides, 3, 4, 5, 

 6, 8 ; and that as regards a fracture-like insertion of the 

 umbilicus, a reputed characteristic feature of Hyalod. 

 Itsvis also, such insertion is found likewise in slides No. 3 

 and 7 specimens of Yarra Yarra, these are matters, therefore. 



