18 
THE VOYAGE OF H.M.S. CHALLENGER. 
admitted in the systematic portion of this Keport. These two groups, although agreeing 
in their essential characters, differ in certain points to which allusion will afterwards be 
made. 
The Head, in all species of Cavolinia, resembles that of Clio ( s . str.). Both tentacles 
are well developed, the right especially attaining large dimensions. 
The Foot is of different form in the two groups. In group A ( Cavolinia trispinosa 
and Cavolinia quadridentata) the fins are like those of Clio (s. str.), as also the posterior 
lobe of the foot, which is long. 
In group B (typical Cavolinise) the fins are not so narrow dorso-ventrally, and they 
form a muscular surface, almost undivided, with the posterior lobe of the foot. This 
latter is very short and almost as broad as the united fins, from which it is scarcely 
separated. 
The Mantle in Cavolinia has a form quite peculiar to the genus, which gives it its 
most striking character, and is reflected in the disposition of the shell. 
The mantle-opening is narrow dorso-ventrally ; morphologically indeed it extends as 
far as the posterior extremity of the lateral slits of the shell, for it is up to this point 
that the opening of the latter extends. The margins of the mantle, however, are 
united together by a narrow transverse membrane, as far as in front of the closing 
apparatus of the shell. These margins are prolonged beyond this united membrane, and 
may extend outwards by the lateral slits in the shell, just as the separate margins extend 
through the anterior aperture of the shell, in such a way as to cover during the life of 
the animal almost the whole external surface of the shell (in the typical Cavolinise, e.g., 
Cavolinia tridentata). 
Besides this in the Cavolinise, in a restricted sense (that is, excluding Cavolinia tri- 
spinosa and Cavolinia quadridentata), there arises between the margins of the mantle 
thus prolonged beyond the uniting membrane, on either side posteriorly, a very extensile 
appendage, which may be double or triple according to the species and according to the 
state of development, and may float out behind. These appendages possibly correspond 
to the two symmetrical appendages on the margins of the mantle of Cuvierina. 
Cavolinia trispinosa and Cavolinia quadridentata, which do not possess these 
symmetrical appendages, are provided on the left side with a lateral lobe (balancer) like 
that of Clio. 
The pallial gland, symmetrical as in all the Cavoliniidse, presents transverse opaque 
and transparent bands of unlike histological nature. The columellar muscle, which is 
rather broad, is situated dorsally as in all the Cavoliniidse, and only directed ventrally at 
the anterior portion where it bifurcates, passing on either side of the oesophagus to be 
distributed to the fins and posterior lobe of the foot. This muscle, however, is not really 
symmetrical, that is to say, situated exactly in the median line ; it is oblique, and this is 
especially visible in Cavolinia longirostris (PI. III. fig. 2), where the insertion of the 
