140 
PACIFIC SCIENCE, Vol. XV, January 1961 
TABLE 3 
Chelyosoma macleayanum 
STATION 
NUMBER 
LENGTH 
OF DISC 
(mm.) 
NUMBER 
OF 
MARGINAL 
PLATES 
ON DISC 
NUMBER 
OF 
GROWTH 
LINES ON 
PLATES 
OF DISC 
23 
— 
7 
— 
23 
— 
7 
— 
36 
3 
8 
0 
23 
7 
8 
— 
44 
8 
7 
2 
37 
8 
8 
2 
44 
8.5 
7 
2 
B51-32 
10 
8 
2 
37 
11 
8 
2 
45 
12 
8 
2 
B51-33 
12 
8 
2 
B51-32 
13 
7 
2 
37 
14 
8 
1? 
37 
17 
8 
3 
B51-31 
30 
8 
3 
indistinct or absent except in largest specimen 
(disc 30 mm. long); 0-3 growth lines present 
on plates of disc. Quantitative information is 
presented in Table 3. 
DISCUSSION: All of the specimens of this 
well-known arctic species obtained at Point Bar- 
row are assigned to the form typica (Arnback, 
1934), which is characterized by possession of 
a single central intersiphonal plate and 5-9 
marginal plates. Van Name (1945) considers 
the individuals with 7 more or less symmetrically 
placed marginal plates (Fig. la) to represent 
the normal condition, and this appears to be 
the case in northern Atlantic waters, but Arn- 
back has described a population from Pitlekaj 
on the Siberian arctic coast (67° 7 ' N., 173° 
24' W.) in which the modal number of mar- 
ginal plates is 8, the 8th plate being located on 
the right as in the Point Barrow specimens. The 
sparse comparative data available ( Table 4 ) sug- 
gest a zoogeographic cline with respect to num- 
ber of marginal plates, and a re-examination of 
earlier collections of C. macleayanum with this 
possibility in mind should prove worthwhile. 
Growth lines on the test plates show a gen- 
eral increase in number with the size of the 
animal (Table 3), and suggest that the largest 
individuals taken were in their 4th year (see 
Huntsman, 1921). 
distribution: Circumpolar arctic: Russia, 
Norway, Iceland, Greenland, eastern N. Amer- 
ica south to Cape Ann, arctic N. America, 
Bering Strait, Sea of Okhotsk, Siberian arctic. 
Chelyosoma inaequale Redikorzev, 1913 
Figs. 2, 3 a-e 
C. inaequale Redikorzev, 1913, pp. 206-208, 
fig. 2; Arnback, 1934, pp. 80-81; Van 
Name, 1945, p. 209. 
DESCRIPTION: Body elongate-oval, the dorsal 
surface flattened to form an oval disc bearing 
the siphons and surrounded hy a marginal ridge 
whose prominence varies with degree of body 
contraction. Both apertures 6-lobed. Tunic 
tough, flexible, translucent, grey-white to yel- 
lowish, locally thickened to form triangular 
plates on all siphon lobes (Fig. 3 b) and a con- 
spicuous array of warts, ridges, and small plates 
on the disc and marginal ridge, giving the dor- 
sal surface a tesselated appearance (Figs. 2, 3 a). 
Internal surface of tunic bearing a ridge below 
Fig. 2. Chelyosoma inaequale. Dorsal view of disc. 
