2S0 report— 1846. 



joint (28C?) inclusive, as the pleur apophysis', it is not so obvious whether 

 the bones 29-32 form a subdivided hcemapophysis, 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 hcemal spine. But the general 

 character of the inverted arch (H in), as the haemal 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 (34-37). These, in the series of fishes, manifest, in as many 

 permanent arrests, the chief phases of development that the corresponding 

 appendages of the occipito-haemal arch have been described to pass through. 

 The diverging appendage of the fronto-haemal 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 

 haemapophysial 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 2sd 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 

 (28^) of the pleurapophysis is usually articulated a bone (24) connecting it 

 with another bone (20) in advance : the ground for regarding 24 as appertain- 

 ing to the arch (20, 21 and 22, H iv) 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 iv, H iv, 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 u, 14, which more 

 immediately support the olfactory ganglions 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 haemal canal being in like manner closed by the approximation of 



