250 REPORT—1846. 
joint (28d) inclusive, as the plewrapophysis: it is not so obvious whether 
the bones 20-32 form a subdivided hemapophysis, or whether the terminal 
bone (32), forming by symphysis with its fellow the crown of the inverted arch, 
may not be the moiety of a mesially divided hemal spine. But the general 
character of the inverted arch (H 111), as the hemal complement of the fron- 
tal vertebra is unmistakeable, and its serial homology with the succeeding 
arches (H 11 and H 1) is fully illustrated in fishes by its supporting diverging 
appendages (31-37). These, in the series of fishes, manifest, in as many 
permanent arrests, the ehief phases of development that the corresponding 
appendages of the occipito-hzmal arch have been described to pass through. 
The diverging appendage of the fronto-hemal arch is a single and simple 
bony style in the lepidosiren ; it consists of three or four simple rays in the 
monk-fish and some other plagiostomes ; it has one ray expanded into a broad 
proximal piece in the conger, which sustains a distal segment of the appendage, 
one member of which, the ‘subopercular,’ still retains the long and slender, 
ray-like form, which is, also, clearly traceable in: the broader but long and 
curved ‘opercular’; in the cod, as in most osseous fishes, the parts of the 
second segment of the appendage (35, 36, 37, fig. 5) are metamorphosed, like 
the proximal one (34), into broad and flat bones. The fin-like fold of inte- 
gument, sustained and moved by means of this diverging appendage and its 
muscles, reacts upon the surrounding water ; but, like the hyoid-fins, with 
which the tympanic or opercular fins are closely connected, they are chiefly 
subservient to the creation of the respiratory currents and their direction 
through the gill-chambers. The weight of these appendages, and the con- 
stant movements in connection with respiration, as well as those which the 
hemapophysial portions of the arch, modified in subserviency to nutrition 
have to perform, as jaws, explain the necessity of the subdivision of the sup- 
porting pedicle into overlapping pieces allowing of a certain elastic yielding 
with recoil, and thus diminishing the liability to fracture without affecting, 
except by increasing, the strength of the arch. The trochlear joint between 
- the two elements of this arch (at 28d and 29) with its cartilage and synovial 
sac, repeats the complex structure of the articulation between the vertebral 
and sternal portions of the ribs in birds. To the fore-part of the lower piece 
(28d) of the pleurapophysis is usually articulated a bone (24) connecting it 
with another bone (20) inadvance: the ground for regarding 24 as appertain- 
ing to the arch (20, 21 and 22, H 1v) will be explained in the description of 
that arch. 
There remains, then, in the fish’s skull, to be considered, the group of 
bones (N tv, H rv, fig.5) forming its anterior extremity; and we have to in- 
quire, whether there can be traced in this easily separable group such a con- 
cordance in its formation with the arrangement of the constituents of the 
foregoing segments as will justify its being regarded as a natural segment of 
the skull, and as still illustrating the type on which all the other segments of 
the endoskeleton have been constructed. Fig. 4 gives the same view of the 
bones of this group in vertebral relation with the rhinencephala as the views 
in figs. 1,2 and 3 do of the bones having a similar relation to the three larger 
segments of the brain: we perceive the single and symmetrical bone (13) 
forming the basis of the arch, and sustaining the bones 14, 14, which more 
immediately support the olfactory ganglicns and transmit their nerves, either 
by grooves or foramina, to the olfactory capsules: the key of the arch is 
formed by the single and symmetrical bone 15, which is articulated to and 
chiefly sustained by the bones 14, 14: but 15 is expanded and deflected 
anteriorly so as to rest directly upon 13 and completely obliterate the neural 
canal ; the heemal canal being in like manner closed by the approximation of 
