134 
PACIFIC SCIENCE, VoL VIII, April, 1954 
TABLE 1 
Characteristics Which Differentiate the Species in the Acanthomysis costata Group 
CHARACTERISTIC 
A. alaskensis 
A. stelieri'^ 
A. cost at a\ 
A. davisi 
Size 
About 20 mm. 
About 20 mm. 
8 mm. 
9-11 mm. 
Rostrum 
Acute; reaching al- 
most to end of ocu- 
lar peduncle 
Not as acute as A. 
alaskensis; reaching 
to middle of ocular 
peduncle 
"short . . . pointed” 
Subacute; not reach- 
ing to middle of ocu- 
lar peduncle 
\ 
Cornea 
Dorsoventrally com- 
pressed; laterally 
broadened 
Rounded (?) 
Rounded (?) 
Rounded to slightly 
compressed; not 
markedly laterally 
broadened 
Antennal squame. . . . 
7 times as long as 
broad ; 2 times 
length of antennular 
peduncle 
7 times as long as 
broad; 1.5 times 
length of antennular 
peduncle 
5 times as long as 
broad; 1.3 times 
length of antennular 
peduncle 
5 times as long as 
broad; 1.3 times 
length of antennular 
peduncle 
Propodi of thoracic 
legs 3-8 
6-8 secondary arti- 
cles 
4-6 secondary arti- 
cles 
4-5 secondary arti- 
cles 
4-5 secondary articles 
Rugae of abdominal 
somites 
1 
1 fold 
2 folds, discontinu- 
ous dorsally 
3 folds 
2 folds, 1st slight 
2! 
1 fold 
2 folds, discontinu- 
ous dorsally 
2 folds 
2 folds, 1st slight 
3 
1 fold 
3 folds, discontinu- 
ous dorsally 
2 folds 
2 folds, 1st slight 
4 
3 folds 
2 folds, discontinu- 
ous dorsally 
2 folds 
3 folds 
5 
3 folds, 2 discontin- 
uous dorsally 
2 folds, 1 discontin- 
uous dorsally 
2 folds 
3 folds, 2 discontinu- 
ous dorsally 
6 
3 folds, all discon- 
tinuous dorsally 
1 fold, continuous 
dorsally 
2 folds, first project- 
ing as triangular 
plate, second con- 
tinuous 
3 folds, all discontin- 
uous dorsally 
Telson 
length/length 6th 
abdominal segment 
2/1 
1.5/1 
2/1 
2/1 
length/breadth at 
base 
3/1 
2.5/1 
2.5/1 
2.5/1 
breadth at tip/ 
breadth at base. . . . 
1/20 
1/6 
1/15 
1/40 
penultimate long 
lateral spines 
Reaching beyond tip 
of telson 
Reaching beyond tip 
of telson 
Not reaching to tip 
of telson 
Reaching about mid- 
length of telson 
posterolateral spines 
Length equal to 
breadth of telsal tip 
Length equal to 
breadth of telsal tip 
Length equal to 
twice breadth of tel- 
sal tip 
Telsal tip so different 
that comparison not 
valid 
4th pleopods of male 
exopod 
basal article 
4.5 times length dis- 
tal article 
Undescribed 
5 times length distal 
article 
4.5 times length dis- 
tal article 
terminal setae 
4 times length distal 
article 
Undescribed 
3 times length distal 
article 
3 times length distal 
article 
* Comparison based on Derzhavin’s figures and short description; the teJson shown by Tattersall (1951: 212, fig. 88) does 
not agree with that shown for the type species. 
i' Comparison based on Holmes’s original description ( 1900: 221) and upon the more adequate redescription by Tattersall 
( 1932: 327). The specimen depicted by Hansen (1913: 177) showed a telson with the terminal spines like A. sculpta 
(Tattersall), not A. costata. 
