On the Brain and Skull of Amphioxus lanceolatus. 229 
more than eight myotomes (and consequently protovertebre) of 
Amphioxus, in addition to those already mentioned, can be reckoned 
as the equivalents of the parachordal region of the skull in the 
higher Vertebrates. Thus it would appear that the cranium of the 
latter is represented by those segments of the body of Amphiowvus 
which lie in front of the fifteenth, counting from before backwards, 
and that their cranial nerves are represented by the corresponding 
anterior pairs of nerves in Amphiowus. 
In all Vertebrata above Amphiovus the nerves which answer 
to the seven posterior pairs in Amphioaus unite into one or two 
trunks on each side, and give rise to the nerves called pneumo- 
gastric and glosso-pharyngeal ; and as these pass out of the skull 
in front of the occipital segment, it would appear that this seg- 
ment is, in the main, the result of the chondrification, with or 
without subsequent ossification, of the fourteenth protovertebra. 
There is no evidence, at present, that the ear-capsule repre- 
sents a modification of any part of the vertebral skeleton, nor that 
the trabecule are any thing but an anterior pair of visceral arches. 
And if these parts have nothing to do with centra, or arches, of 
vertebrie, it follows that the numerous protovertebree which lie 
in front of the fourteenth in Amphiowvus, are represented only 
by muscles and nerves in the higher Vertebrata. 
The anterior end of the cerebro-spinal axis of Amphiowus 
answers to the lamina terminalis of the thalamencephalon of the 
higher Vertebrata, the cerebral hemispheres and olfactory lobes 
remaining undeveloped. 
If the auditory nerve is, as Gegenbaur has suggested, the dorsal 
branch of a single nerve which represents both the portio dura 
and the portio mollis, the auditory organ of Amphioxus is to be 
sought in connexion with the dorsal branch of its eighth nerve. 
I have found nothing representing an auditory organ in this posi- 
tion; and I can only conclude that Amphiowus really has no 
auditory apparatus. In all other respects, however, it conforms 
to the Vertebrate type; and, considering its resemblance to the 
early stages of Petromyzon described by Schultze, I can see no 
reason for removing it from the class Pisces. But its perma- 
nently segmented skull and its many other peculiarities suggest 
that it should be regarded as the type of a primary division or 
subclass of the class Pisces, to which the name of Hntomocrania 
may be applied, in contrast to the rest, in which the primary 
segmentation of the skull is lost, and which may be termed 
Holocrania. On a future occasion I propose to show in what 
manner the skull of the Marsipobranch is related to that of the 
higher Vertebrata, and more especially to the skull of the Frog 
in its young tadpole state. 
EXPLANATION OF THE FIGURES. 
A, C, D are diagrammatic, but accurate, representations of the anterior part 
of the body in Amphiorus (A), in an Ammocete 1-6 inch long (C), and 
in a fully grown Ammocate 5:7 inches long (D). B is a copy of the 
Ann. & Mag. N. Hist. Ser. 4. Vol. xv. 16 
