Zoogeographical Investigations in Southern Greenland. 253 
Spirontocaris may be distinguished by the fact that the first-named 
has only 5 pair of natatory rami (exopodites), viz.: on mxp. 1—3 and 
pl—2, whereas Spirontocaris has 7 pairs, the additional two being on 
p3—p4. This agrees entirely with my own results; I am unable to see, 
however, whence Sars has his knowledge of Spirontocaris, if not from 
the spec. of Sp. Cranchii and Sp. pusiola hatched by him (Sars l. c¢. 
on Hippolyte var.), as the only description of an older Spirontocaris 
larva which I have been able to find in extant literature is the larva 
described by Craus (I. c.); this lacks exopodites on p3—p4, and I am 
not certain that it really belongs to this genus. 
Being unable to determine my larvae with certainty, I have num- 
bered them for convenience of reference. I have, however, disregarded 
Nr. 5 in the following remarks concerning developement, as I am not 
certain that it really belongs to this genus. 
In point of habit, the larvae resemble that of Hippolyte varians 
described by Sars, but may be distinguished from this at a first glance 
by being more slender, and by the fact that none of them have any 
dentition on the anterior part of the lower edge of the carapace. There 
is generally a small protuberance behind the basis of the rostrum; 
the length of the rostrum does not generally exceed that of the eyes, and 
frequently falls short of this. All the larvae have a supra-ocular spine. 
It is remarkable that the larvae of the different species come into 
the world at very different stages. Nr. 1, for instance (first free stage 
of Sp. polaris?), im which the 6. and 7. caudal segments are not yet 
separated, and the uropoda still within the integument, has nevertheless 
comparatively well-developed pleopoda and well articulated pereiopoda 
of almost adult form but without expodites at all. Larva Nr. 3 on the 
other hand, has in its youngest known stage the 6. and 7. caudal seg- 
ments separate, and the uropoda free, while p3—pd have no trace of 
articulation, and pleopoda are lacking. 
This difference in the degree of developement cannot be taken as 
proof that the larvae do not all belong to the same genus, and that some 
have consequently been incorrectly determined ; it should rather be regar- 
ded as a secondary feature, due to the partially arctic conditions, possibly 
arising from the low salinity of the water nearest the surface. Arrested 
metamorphosis is, moreover, also known among not a few arctic forms, 
the Echinodermata especially, and one species of Sclerocrangon, Scl. 
boreas, has, as far as we know (teste Sars, vide supra p 246) no larval 
stage at all, while the other, Sc. ferox, has larval developement, although 
arrested. 
Spirontocaris has, it would seem, no real Zoea stage; Larva Nr. 1 
should, it is true, from the shape of the telson, be a Zoea, but the pereio- 
poda are here too well developed, and a true Zoea has, moreover, no 
pleopoda. Most of the larve should be characterised as Mysis-stages. 
In the young stages, the mandibles are of adult form, a distinct 
