The Polyclad Genus Pseudoceros, with Special Reference to the 
Indo-Pacific Region 
Libbie H. Hyman^ 
The occasion of this article is the acquisition 
of three species of the polyclad genus Pseudo- 
ceros from the region of the Coral Sea. Pseudo- 
ceros is one of the most populous polyclad 
genera. In 1950, Marcus (1950: 84-88) listed 
79 names in this genus, but the elimination 
of synonyms and of species that should be 
transferred to the genus Acanthozoon because 
of their papillate dorsal surface reduces the 
list to 72. The list, however, omits seven 
Pseudoceros species described from Japan by 
Stimpson (1855: 380; 1857: 25-26) and ac- 
knowledged as valid by Kato (1944: 298). 
Marcus further left out of consideration six 
more of the Kelaart-Collingwood species that 
are figured in color by Collingwood (1876: 
90-97) and that probably belong to Pseudo- 
ceros. One of these, Eurylepta affinis, has been 
refound in the Philippines and is validated as 
a species of Pseudoceros by Stummer-Traunfels 
(1933: 3566). To these must be added two 
species described by Kato (1944: 301), three 
by Marcus (1949: 86; 1950: 81; 1952: 91), 
and four by me (Hyman, 1953: 363-370). I 
have further shown that Amhlyceraeus luteus 
Plehn, 1898, must be transferred to Pseudo- 
ceros (Hyman, 1953: 366). One must further 
note that Stummer-Traunfels (1933: 3565) 
gave colored figures of 11 more species of 
Pseudoceros but did not name them; three of 
these came from the Siboga collections in the 
Netherland East Indies, six were taken by 
Semper in the Philippines, and one each came 
from Zanzibar and Batavia. Clearly there are 
at least 100 species of the genus mentioned 
in the literature. 
1 American Museum of Natural History, New York. 
Manuscript received June 19, 1953. 
The genus is characteristic of tropical and 
subtropical waters and appears to center in 
the Indo-West Pacific region. Two species are 
known from the Gulf of California and two 
from the California coast (Hyman, 1953: 363- 
370). Pseudoceros canadensis (Hyman, 1953: 
368), from British Columbia and since found 
abundantly in Puget Sound, is a surprising 
exception to the usual limitation of the genus 
to warmer waters. This appears to be the most 
northerly record for the genus. Plehn (1896: 
171) reported the occurrence of the Mediter- 
ranean species P. splendidus Stummer-Traun- 
fels, 1933 ( = P. superbus Lang, 1884) in the 
Galapagos Islands, but the identification ap- 
pears dubious. In the western tropical Atlan- 
tic, four species were reported from Bermuda 
by Verrill (1900: 596; 1901: 42) of which ' 
three are endemic and one is the Mediter- 
ranean P. splendidus. Hyman (1939: 17) added 
another Bermudan species and reported the 
refinding of splendidus and of two of VerrilPs 
species at Bermuda. Surprisingly there are no 
records of the genus in the West Indies and 
Caribbean except for P. splendidus at Puerto 
Rico (Hyman, 1954). The species P. macu- 
losus Pearse (1938: 85) from the Gulf of Mex- 
ico is stated to have a papillate dorsal surface 
and therefore should presumably be trans- 
ferred to the genus Acanthozoon. Three spe- 
cies have been described from the Brazilian 
coast by Marcus (1949: 86; 1950: 81; 1952: 
91). Three species are recognized by Lang 
(1884: 538-542) from the Mediterranean, but 
there seem to be no other findings in the 
eastern tropical Atlantic. One species de- 
scribed by Palombi (1928: 605) from the 
Suez Canal, three by Meixner (1907: 465- 
473) from French Somaliland, and 22 listed 
219 
