Arthressa evansi , n. sp. — K ay 
113 
FIG. 1. Arthressa evansi. Drawn from the living 
animal. A. Dorsal view; B, ventral view. 
DISCUSSION 
Of the three species now included in Ar- 
thressa, A. evansi appears to be intermediate in 
characters between A. cincta and A. elioti. A. 
■evansi resembles A. cincta in coloring and in the 
smooth mantle edge; it resembles A. elioti in 
the length of the anterior aperture and in the 
rounded body whorl. 
Although Evans (1950) suggested that Vol - 
vatella fragilis Pease should be retained as it 
was sufficiently described, no collections of this 
species have been recorded in Hawaii since the 
original description. Pease (I860) merely de- 
scribed the type specimen as from the "Sand- 
wich Islands,” and the animal was illustrated as 
being white. Evans (1950) distinguished Vol- 
vatella from Arthressa on the basis of Peases 
(I860) description of the anal opening into 
the posterior spout in Volvatella; the anus opens 
on the dorsal surface of the visceral mass in 
Arthressa . If specimens agreeing with Pease’s 
(I860) description of Volvatella, including a 
posterior anus, are collected in the future, Evans’ 
distinction between the genera will merit rec- 
ognition; however, it is possible that Pease’s 
description was misleading, and the pore which 
he described as the anus may have been the pore 
of the gland which secretes the viscid fluid 
which these animals emit. If this is the case, 
Evans could have gone further and entirely 
suppressed the genus Volvatella of Pease. 
Evans (1950) described Arthressa as a sac- 
coglossan genus, and it should therefore be re- 
moved from the position assigned Volvatella 
in the Akeratidae of Thiele (1931). Arthressa 
is of interest in the scheme of opisthobranch 
relationships in that it is a suctorial form feed- 
ing on various algae. The occurrence of a shell 
and a generalized mantle cavity and alimentary 
canal suggest that Arthressa holds a position 
in the Saccoglossa analogous to that of Actaeon 
(Fretter and Graham, 1954), which also pos- 
sesses many prosobranchiate characters. 
REFERENCES 
Evans, T. J. 1950. A review of Pease’s genus 
Volvatella, together with a preliminary re- 
port on a new saccoglossan genus. Proc. Malac. 
Soc. London 28: 102-106. 
Fretter, V., and A. Graham. 1954. Observa- 
tions on the opisthobranch mollusc Actaeon. 
Journ. Mar. Biol. Assoc. U. K. 33(3): 565- 
585. 
Nevill, G., and H. Nevill. 1869. On some 
new marine Gastropoda from the Southern 
Province of Ceylon. Journ. Asiatic Soc. Ben- 
gal 38, pt.2: 65-69. 
Pease, W. H. 1860. Descriptions of new species 
of Mollusca from the Sandwich Islands. Zool. 
Soc. London Proc. 28: 18-37. 
1868. Descriptions of marine Gastrop- 
oda inhabiting Polynesia. Am. Journ. Conch. 
4: 71-80. 
Thiele, J. 1931. Handbuch der systematischen 
W eichtierkunde. Verlag von Gustav Fisher, 
Jena. Bd. I, 37 6 pp., 470 figs. 
