22 W. M. TATTEKSALL. 



the genera of tlie family before him, but drew up his definition from the descriptions 

 and figures of Willemoes-Suhm (1875). He includes in the family the single type 

 genus, Petalophthalmus, Will.-Suhm, with two species — P. armiger, Will.-Suhm, and 

 P. willemoesii, a new species which he founds for the reception of the female ascribed 

 by Suhm to P. armiger. 



Faxon and Hansen, however, have since pointed out that the female specimen 

 described by Will. -Suhm is in reality a Boreomysis, probably B. scyphops, G. 0. Sars, 

 so that if this latter view of its identity be adopted, P. willemoesii becomes a synonym 

 of B. scyphops. 



Czerniavsky's definition of the family is inadequate, inasmuch as no reference is 

 made therein to the remarkable characters of the carapace, and the first and second 

 thoracic limbs, while the importance which is given to the supposed characters of the 

 exopods of the thoracic limbs is exaggerated, the difference in development as 

 compared with those of the Mysidse being very slight. 



The definition given by Holt and Tattersall, 1906 (2), may, therefore, be adopted 

 with some slight alteration in the characters ascribed to the eyes, rendered necessary 

 by recent discoveries. 



Examination of British specimens of Hansenomysis fyllse (Hansen, 1887) has 

 revealed the fact that eyes are, in reality, present in this species. They resemble those 

 described below for the Antarctic species, except that the lappets are much reduced 

 and almost obsolete. 



Further, in a new species of Petalophthalmus, P. oculatus, recently defined by 

 lUig (1906), the eyes are described as well developed, with the cornea bright brown in 

 colour and distinctly facetted. 



In the amended definition of the family, therefore, the description of the eyes 

 would read : — Eyes (first cephalic appendages) small, either imperfectly developed as 

 lamellar or spiniform organs, without visual elements, or furnished with a distinct 

 cornea in which visual elements are clearly defined and functional. 



Genus Hansenomysis, Stebbing. 



Arctomysis, Hansen, 1887 {non Czerniavsky, 1883). 



Hansenomysis, Stebbing, 1893. 



Hansenomysis, Holt and Tattersall, 1906 (1 and 2) ; Tattersall, 1907. 



The name Arctomysis, given to this genus by Hansen (1887), having been already 

 used by Czerniavsky (1883) for an entirely different form, was changed to 

 Hansenomysis by Stebbing (1893). Arctomysis Czerniavsky is itself a synonym of 

 Boreomysis G. 0. Sars. 



Of the other three genera belonging to the Petalophthalmidse — Petalophthalmus, 

 Ceratomysis and Scolophthalmus, Hansenomysis comes nearest to the last. Both 

 agree in having the first thoracic limbs devoid of exopods and lacking the internal 

 lamelliform meral lobe, and in the presence of well-developed exopods to the second 



