4 RICOLLETT M.-N. KI. 
And as, moreover, a few species have, likewise, been shewn to occur 
in the great depths of the Central Atlantic (from 1200 to 4000 metrès) 
almost under the Equatorl, it will be seen that, in all probability, this 
genus is spread throughout from Pole to Pole, in suitable depths where 
a uniform (low) temperature prevails. 
Whilst the number of observed species is already quite great, the 
materials which are at present to hand respecting their detailed charac- 
teristics are, as a rule, entirely inadequate, and, it may’ be said, that 
scarcely one of the many species is, as yet, known in all the stages of 
its development from infancy to maturity. 
As is known, most of the species during their growth undergo very 
considerable changes in their colouring, scaly covering, and also in the 
structure of the body, but the number of specimens which have been 
obtained in each of these instances have, virtually, almost always been 
few. Of some forms there may thus have been found but one, or a few 
young individuals, and from these the characteristics of the species have 
had to be derived, even when the type specimen has been but a newly 
hatched fry?; and in instances in which a dredge or trawl has brought 
up several at a time, such have, as a rule, all been of a like age and 
stage of development. Some of these species, likewise, are only to be 
met with in poorly preserved specimens, obtained in some instances from 
the stomachs of sharks and seals. 
To find the connection, from the scanty materials, between the appar- 
ently multitude of forms of this genus, which, by degrees, have been 
brought to light, and of which one stage of development almost invariably 
differs from the succeeding stage, at the same time that individuals of a 
like stage of development can exhibit considerable differences in their 
colouring and covering of scales, is still, in most cases, difficult or impos- 
sible. In this respect Zycodes belongs to the least revealed of all genera 
of Arctic fishes, and barely two Authors agree on the characteristics or 
definition of the species. It is, moreover, therefore necessary to remember 
«that, after all, the Zycodes species cannot be defined from specimens of 
immature size». 
I Vaillant, Exped. Scientif. Travailleur Talisman 1880—83, Poissons, p. 306—312, PI. XXVI, 
Fig. 1—2 (Paris 1888). 
2 Thus Z. perspicillum, Kr. 1844, L. rossit, Malmgr. 1864, Z. gracilis, M. Sars 1866, 
L. sarsii, Coll. 1871. 
8 Lütken, «Grønlands og Islands Lycoder» in Vidensk. Medd. Naturh. Foren. Kbhvn. 1879— 
80, pP. 22. 
