Mysidacea and Euphausiacea — BANNER 
to 8 secondary articulations. Armature of 
basal plate of exopod variable. 
Terga of first live abdominal somites bear- 
ing transverse folds somewhat similar to those 
described iot Acanthomysis davm^ 2 smti ( 1948;^ : 
96). Sixth abdominal segment with three folds 
that are discontinuous dorsally; anterior pair 
set on a slight angle to transverse line of body 
and poorly developed; posterior two pairs 
transverse to body and well developed. 
Telson slightly over twice as long as the 
sixth abdominal segment, triangular in shape 
with anterior lateral margins concave, middle 
and posterior lateral margins straight to 
slightly convex. Telson 3 times as long as 
broad at base, with breadth at tip one-twen- 
tieth breadth of base. Spines on basal half of 
lateral margins more or less regularly placed 
and of subequal lengths; larger spines of pos- 
terior half of lateral margins separated from 
each other by groups of 3 to 5 much shorter 
spines; penultimate lateral spines with tips 
reaching beyond tip of telson; final spines 
very slightly longer than tip is broad and 
separated by 2 very short spines. 
Inner uropod reaching almost to end of 
telson and bearing 4 to 5 spines near statocyst; 
outer uropod 1.5 times length of inner. 
Only fourth pleopods of male biramous, 
with tip of exopod reaching slightly beyond 
end of fifth abdominal segment. Proximal 
article of exopod 4.5 times length of distal 
and bearing a single strong spine; distal article 
bearing 2 strong spines that are 4 times length 
of distal article. Fifth pleopod of males 0.8 
as long as fourth. 
SPECIMENS examined: Type specimen: A 
submature male, 19.7 mm. long, collected by 
theKing Crab Investigation, haulC-117, June 
8, 1941 , from the stomach of Theragra chalco- 
gramma (Pallas) (Alaskan pollock), taken off 
Cape Seniavin on the Alaskan Peninsula 
(about 160°W.) in 23-26 fathoms. U. S. Na- 
tional Museum 95638. 
Paratypes : Many others collected with type, 
some in semidigested condition. Five others 
also collected by the King Crab Investigation 
133 
at Point Moller, Alaska, from the stomachs 
of cod. 
DISCUSSION: These specimens were inter- 
preted to be immature on the basis of char- 
acteristics of the female, not the male, for the 
females of size similar to the type did not 
have their oostegites fully developed. On the 
other hand, it may be that the males are ma- 
ture but reach a smaller size at maturity than 
do the females. 
This species belongs to the group in this 
genus that have the abdomens with dorsal 
folds in the chitin: A. mitsukurii 
(I9IO: 250), A. stelleri (Derzhavin) (1913: 
202), A. davisi Banner (1949: 95), A. costata 
(Holmes) (I9OO: 221), H. sculpta (Tattersall) 
(1933: 197), and A. borealis (described in this 
paper) . A. mitsukurii can be distinguished from 
this species and from all other species in the 
group by the presence of small denticles on 
the folds of the abdomen, as well as by other 
characteristics. A. sculpta can be distinguished 
by the presence of four large spines on the 
tip of the telson and by the lack of a strong 
bristle on the basal article of the exopod of 
the fourth pleopods of the male. A. borealis 
is easily separated by the rounded tip of the 
telson and by the uniform series of spines on 
its distal tenth. 
However, this species and the remaining 
three species, A. costata, A. stelleri, and A. 
davisi, are most obviously closely related. A. 
costata and A. davisi are both smaller species 
found in more southern waters, whereas A, 
stelleri and A. alaskensis are larger northern 
forms. The characteristics that will serve to 
separate them are given in Table 1. 
From the examination of this table it will 
be seen that the species have much in com- 
mon and that the diagnostic characteristics 
—primarily the rostrum, eyes, antennal squa- 
me, abdominal grooves, shape, and armature 
of the telson — are small differences in propor- 
tions in parts that are known in other species 
to be variable. The fourth pleopods of the 
male show less difference than is found be- 
tween the northern and southern races of 
Neomysis rayi. 
