64 KINSHIPS OF CYCLOSTOMES 
of the group. Among its primitive features are to be in- 
cluded: skeleton and muscles, continuous vertical fin, gill 
characters (p. 260), viscera (p. 263), urino-genital organs 
(pp. 266, 270), nervous and circulatory systems (pp. 260, 
269, and 274). With these must be taken into account: 
absence of mandible* and of paired fins and girdles; and in 
addition the remarkable conditions of metamerism (p. 14). 
Little more that a vague kinship between lampreys and 
fishes has been established by the study of living forms. 
And, on the other hand, it would appear equally impracti- 
cable to obtain evidence bearing upon this problem from 
the side of palzontology. All that is known of the recent 
Cyclostomes more than suggests that their soft body struct- 
ures would prove most unfavourable to fossilization. It 
would be only, therefore, in the event of some of their 
ancient members possessing calcified structures that palae- 
ontology would be able to offer a clue as to their ancient 
affinities. 
Upon the problem of their descent the evolution of 
fishes has, however, an undoubted bearing, in suggesting 
the lines and effects of aquatic evolution and the perma- 
nence of generalized types. It certainly tells of the ex-— 
treme slowness of the evolution of aquatic forms and con- 
vinces us that the ancestral Cyclostome could only have 
occurred in a time stratum exceedingly remote. Palzon- 
tology cannot perhaps hope to obtain more than sugges- 
tions of the ancestral forms, although these, from their 
generalized characters, may well have survived during geo- | 
* The cartilages of the mouth region of Cyclostomes have been homologized 
with the structures of gnathostomes; Pollard recently (Amat. Ams. ix, pp. — 
349-359) ascribes a cirrhostomial origin to the mouth parts of a Teleostome 
(catfish), which the writer cannot believe has been demonstrated; variations 
in the number, shape, and function of the cartilages of the mouth rim of 
Cyclostomes might well have occurred within the limits of this ancient group. 
