A New Opisthobranch Mollusc from Hawaii 
Alison Kay 1 
The opisthobranch genus Arthressa was pro- 
posed by Evans (1950) on the basis of a revi- 
sion of the genus Volvatella Pease I860. Pease’s 
genus was known only from four species, three 
described by Pease (1860, 1868), based on sin- 
gle specimens from the Pacific, and a fourth 
described by G. and H. Nevill (1869) from 
Ceylon. Evans (1950) pointed out several dis- 
crepancies in Pease’s species descriptions, and, 
while retaining V. fragilis Pease, the type species, 
he proposed the genus Arthressa to include A. 
cincta (G. and H. Nevill) from Ceylon and A. 
elioti Evans which was newly described from 
Zanzibar. 
Four specimens of a species congeneric but 
distinct from A. cincta and A. elioti have been 
collected in the Hawaiian Islands, and are here 
described as a new species. 
GENUS Arthressa Evans 1950 
SHELL: Thin, calcareous, covered by a thicker 
periostracum extending beyond the calcareous 
margin. Body whorl ovate, broadest in the mid- 
dle, and contracting posteriorly to a spout. An- 
terior aperture as broad as shell in its anterior 
half. Right lip overlapping reflected left lip be- 
hind aperture; left lip depressed behind margin 
and passing at the junction of right and left lips 
under recurved left margin into a deep umbilical 
cleft; margin continuous with the spout. On left 
side of spout columella of a sunken spire of 
approximately three turns visible through per- 
iostracum. 
ANIMAL: Tentaculate area of head rimless, 
frontal, and oriented ventrally. Dorsal tentacles 
short and smooth. Lateroventral tentacles deeply 
grooved laterally, slightly bilobed at tip. Eyes 
immediately posterior to grooves. Anal opening 
on dorsal surface of visceral mass beneath man- 
1 Department of General Science, University of Ha- 
waii. Contribution No. 144 from the Hawaii Marine 
Laboratory. Manuscript received July 29, 1959- 
tie. Foot small and triangular with blunt cor- 
ners. 
TYPE SPECIES: A . cincta (G. and H. Nevill). 
Ceylon. 
Arthressa evansi, new species 
Fig. 1 
HOLOTYPE: Bishop Museum, no. 8901. 11 
mm. in length; 6 mm. in breadth; collected in 
a tidepool, Diamond Head Beach Park, Oahu, 
Hawaii, November, 1956. Paratype: Bishop Mu- 
seum, no. 8902. 11 mm. in length; 5 mm. in 
breadth; collected with holotype. 
Length of anterior aperture more than half 
that of shell. Body whorl rounded, not flattened 
on the left, and with the spiral curve of the 
shell continued into it. Length of spout less 
than half shell breadth. Opening of spout tri- 
angular with a ventral slit continuous with 
aperture. Mantle edge smooth in spout. Animal 
orange, freckled with a darker shade, and with 
a band of freckling crossing body whorl. Foot 
similar in coloring to that of body but freckling 
of a lighter shade. Foot bluntly rounded, not ex- 
tending anterior to head, and with a groove 
separating anterior two-thirds from posterior 
third. Animals extruded a viscid white sub- 
stance from both posterior spout and anterior 
aperture. Specimens collected vary from 8-11 
mm. in length and from 4-6 mm. in breadth. 
The species has been named for the late J. 
T. Evans. 
The specimens were collected in a tidepool on 
the reef flat of Diamond Head Beach Park, 
Oahu. They appeared to be associated with the 
algae Padina and Gracilaria. The mode of pro- 
gression on a hard substrate is such that the 
anterior end of the foot extends forward and 
attaches to the substrate while the posterior por- 
tions move up. The animals readily suspend 
themselves upside down on the surface of the 
water, progressing as waves of muscular con- 
traction pass across the foot. 
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