256 SCIENTIFIC RESULTS OF THE 
the middle, with its free margin tuberculated or digitate. The frontal 
veil is continued ina more or less tuberculated ridge, that limits the true 
back, and posteriorly ends in a single strong or in several smaller 
dorsal (branchial) appendices on the outside of and behind the region 
of the gill. The true back with longitudinal rows of more or less 
developed connected tubercles, sometimes forming low longitudinal 
ridges. The number of leaves in the club of the rhinophoria is not 
large. The gill is composed of a moderate number (five to seven) of 
leaves, which are either simply pinnate or composite (bi- or tripinnate). 
The tentacles are small, flattened or auriculate. The jaws or man- 
dibular plates in form somewhat recall those of the olidiide, strong, 
flattened, sometimes with a peculiar superior process. The rhachis of 
the radula naked; on the pleuree two large hook-formed lateral teeth, 
of which the outer is much larger than the inner; at the outside of 
the laterals are four to eight, somewhat flattened uncine. <A large 
prostate gives the genital apparatus a particular feature ; the arma- 
ture of the penis is of the usual kind. 
About the biological relations of Polycera very little is known, as 
usual among the Nudibranchiata. The spawn of the most common 
northern species is known, and a part of the developmental history 
has been investigated by Ray Lankester.! 
A small number of species have been described by different authors 
in the course of years. Alder and Hancock (Monogr. part 7, 1*55, 
p- 45, XVIII) established and rather well characterized two groups 
of Polycera ; according to these authors Gray soon after (Guide I, 
1857, p. 213) denominated these groups Polycera (typical) and Palio, 
which perhaps might be conserved as subgenera. 
I. POLYCERA (stricte). 
Margo limbi frontalis digitatus. Folia branchialia simpliciter pin- 
nata ; appendices dorsales (branchiales) singula majores. 
Lamelle mandibulares processu superiori aleformi. 
1. P. quadrilineata (O. F. Miller). M. Atlanticum ; Mediterraneum. 
2. P. horrida, Hesse. Journ. de Conchyliol.,3§., XIII, 4, 1878, p. 345. M. 
Atlanticum, 
! Ray Lankester, Contrib. to the Developm. hist. of Moll., Philos. 
Trans., MDCCCLXXY, p. 29, Pl. 10, f. 1-9. 
Meyer and Moebius have, moreover, given a figure of the shell of the 
embryo of their Pol, ocellata (1. ¢., fig. 10). 
