76 THE NAUTILUS. 



the head and neck; the light-colored opalescent line below this 

 area; the interior of the cerata may be almost black and they 

 have a white line on their outer border; the cerata in the dark 

 varieties have an orange tip; the two lateral light lines in the 

 tail region; and the light opalescent line along the margin of the 

 foot. 



These points are taken from O' Donoghue and arranged in the 

 order in which they are dealt with in Eschscholtz and I think 

 it will be obvious at once that such a closeness of description 

 makes it certain that the same species is under consideration in 

 both cases. If only the intervening observers had given a 

 closer account of the color of the living specimen, I think the 

 identity of Hermissenda opalescens with Cavolina crassicornis would 

 have been established earlier. Examination of the radula 

 shows that Bergh, Cockerell and Eliot and 0' Donoghue were 

 all dealing with the same species. The name opalescens, there- 

 fore, must be discarded in spite of its familiarity and of the fact 

 that it describes the characteristic opalescent appearance of the 

 lines of this beautiful species so well, and the older name crassi- 

 cornis substituted for it. 



The classification and synonymy of this form is, therefore, as 

 follows: 



Family: ^Eolidise Eliot, 1910. 



Genus: Hermissenda Bergh, 1878. 



Species: HERMISSENDA CRASSICORNIS (Eschscholtz, 1831). 

 Aeolis (Flabellina ?) opalescens Cooper, 1862. 

 Flabellina opalescens Cooper, 1863. 



Hermissenda opalescens Bergh, 1879; Cockerell, 1901; Cock- 

 erell and Eliot, 1905; O'Donoghue, 1921; O'Donoghue 

 and O'Donoghue (in press" 1 . 



LIST OF REFERENCES. 



1. Bergh, R. Beitr. zur. Kenntniss den Aeolidiaden, vi. Verh. 



d. K. Zool-bot. Ges. in Wien xxviii, 1878, p. 578. 



2. Bergh, R. "On the nudibranchiate Gasteropod Mollusca 



of the North Pacific Ocean, with special reference to 

 those of Alaska," Pt. I. Proc. Ac. Nat. Sci.. Philadel- 

 phia, Ft. I, 1879. 



