Anthessius — Illg 
339 
unusual among poecilostome groups in general. 
Reaching systematic conclusions in the genus 
Anthessius is difficult. The descriptions of many 
of the species are fragmentary or deal with de- 
tails which are not readily comparable with those 
furnished in still other descriptions, so that the 
relatively subtly differentiated species become 
difficult to characterize as distinctive, although 
the information available convinces the tax- 
onomic worker of the specific separation of the 
forms. Some characteristics which seem to serve 
for grouping species are discussed below. Finally 
a very rough key is presented to serve the func- 
tion of demonstrating the specific separation of 
the new species described. As most of the fea- 
tures treated in the key are drawn from the 
literature, some of which is unsatisfactory for 
the details involved, it is to be regarded only as 
a synopsis, not as a reliable device for routine 
identification of species. 
In general, the fifth leg of species is usually 
described and depicted with considerable detail 
and care. Grouping on the basis of this append- 
age should be fairly reliable. In A. solecurti 
Della Valle, the structure as figured and de- 
scribed is unique for the genus and readily serves 
to differentiate the species. Among the remain- 
ing species there are two general types of con- 
tour seen in the fifth leg of females. (The male 
of A. investi gat oris Sewell 1949 has to be in- 
volved in this discussion, as the female of the 
species is not known.) In most species, the 
lateral margins of the free segment of the fifth 
leg are essentially parallel, although there may 
be some constriction at the base or toward the 
apex. In all, the appendage is narrow, so that the 
ratio of length to greatest width is much more 
than 3, ranging from about 3.2 to 4.3 or more. 
In the fifth leg of the remaining species the free 
segment is much broader and more platelike. 
The margins tend to expand and the armature 
may be concentrated on or near a broad distal 
margin. In such forms the ratio of length to 
width of the free segment is much less than 3, 
ranging from about 1.8 to 2.7, as discernible 
from figures accompanying descriptions in most 
cases. A. navanacis (Wilson), which is assign- 
able to this group, is also readily separated from 
all members of the genus by the distinctive out- 
line of the fifth leg, with marked basal constric- 
tion. Few anatomical details are available for 
A. pleurobrancheae Della Valle, but in the de- 
scription the structure of the fifth leg is stressed, 
and this species is unique among those falling 
into the category just described by completely 
lacking marginal ornamentation of spinules on 
the free segment. As remarked above a useful 
dichotomy exists with regards to the armature 
of the fourth swimming legs, but this detail is 
not available from all species descriptions. Fi- 
nally, there are readily determinable groupings 
available based on the ratio of length to width 
of caudal rami, and in most descriptions in- 
formation on this feature or fairly good illus- 
tration is provided. The degree of variability of 
this character in any given species remains to 
be determined. 
key to species of Anthessius 
Based on adult females, but including A. 
investigatoris Sewell 1949 on the basis of the 
male. A. pleurobrancheae Della Valle 1880 has 
been included, after the treatment of Stock 
(1959), although the original description was 
very scanty and Claus’ account ( 1889) does not 
extensively supplement Della Valle’s. Species 
recently described (Humes, 1959; Stock, 1959) 
have been included in the key, but were not 
treated in the discussions in this paper. 
la. Body habitus very modified, cephalo- 
some much expanded in outline, tho- 
racic segments with widely extending 
epimera, caudal rami elongate, lami- 
nate fitchi, new species 
lb. Body in general approaching typical 
cyclopoid habitus 2 
2a. Ratio of length to width of terminal 
segment of fifth leg more than 3 3 
2b. Ratio of length to width of terminal 
segment of fifth leg less than 3 12 
3a. Terminal segment of exopodite of 
fourth leg with 4 spines 4 
3b. Terminal segment of exopodite of 
fourth leg with 3 spines.. 9 
4a. Ratio of length to width of caudal ra- 
mus about 1 brevifurca Sewell 1949 
4b. Ratio of length to width of caudal ra- 
mus much more than 1 5 
