414 Reviews—Halls Canadian Graptolites. 
discoveries. Prof. Hall here describes fifty-three species, and pro- 
poses six genera, which are either new, or known only from the de- 
scriptions in the Report to which we have alluded. 
Graptolithus Logant may be taken as the type of the great 
novelty in the volume—the discovery of a corneous disk sup- 
porting the slender hydrosoma of the Graptolite at its origin. The 
hydrosoma of G. Logani consists of from eighteen to twenty-three 
simple branches, originating in a symmetrical series on either side 
of the simple primary base. ‘The central disk, which in this species 
is about the size of a florin, envelopes the bases of the branches, 
and these do not bear hydrothecz until they pass the margin of the 
disk. Prof. Hall considers that the disk is composed of two laminz, 
which at least in the central portions are not conjoined, and the 
space was probably occupied by some portion of the animal body. 
He further considers that this structure is obviously adapted to give 
strength and support to the bases of the branches of the hydrosoma, 
and that it perhaps served other purposes in the animal economy. 
He does not refer to any analogous structure among recent animals, 
nor do we know anything among the Hydrozoa at all corresponding 
to it; but it agrees better with Ccelenterate structure than with 
Polyzoan, and so confirms the opinion now generally entertained that 
the Hydrozoa are the modern allies of the Graptolites. — 
The structure exhibited in G. Logani is considered by Prof. Hall 
as the perfect condition of the fragments of the monoprionidian species 
which have been hitherto discovered in Europe as well as in America. 
We cannot concur in this view. All the American species may be 
fragments of such compound individuals, but there are species found 
in Europe which could not be united into such a compound body. 
G. colonus of Barrande, as figured in his ‘Graptolites de Bohéme’ 
(pl. ii. fig. 5), represents a zoophyte in which the older portion is 
free, and the growing end has the ccenosare prolonged, and without 
hydrothecz developed upon it. We are satisfied that this is a faith- 
ful representation of the species, as we have observed a similar struc- 
ture in G. sagittarius, G. Sedgwickii, and an undescribed species—all 
of them from the Silurians of the South of Scotland. The older or 
proximal extremity of these species is always slightly recurved, and 
_ the hydrothece on this part are smaller than on the newer portion. 
If they are fragments of a compound organism, it must have been 
very different from that of G. Logant. A similar difficulty would 
prevent such forms as Didymograpsus Forchhammeri, figured by 
Baily in his paper on Irish Graptolites, being united into compound 
organisms. And there is yet another form of the monoprionidian 
type which it would be as impossible to combine: we refer to 
Cladograpsus linearis of Carruthers, figured in his paper on the 
Graptolites of Dumfriesshire. While, therefore, the Canadian Sur- 
veyors have added a singular and novel type to those hitherto known, 
there is no satisfactory ground for considering that the fragments 
of the Old-World monoprionidian species belong to such a compound 
form. We prefer, with Mr. Salter, to consider the group represented 
by G. Logani as forming a new genus, to which he has given the 
name of Dichograpsus. 
