276 report— 1846. 



centres of the vascular system (heart and lungs) ; according to which types, 

 for example, it may be either closed below by the meeting of the sternal ribs 

 (haemapophyses) or by the intervention of a single or divided sternal bone 

 (haemal spine). And, further, since in fishes, as the lowest class of vertebrata, 

 the vegetative character of repetition of forms, proportions and composition 

 in the successive segments of the skeleton prevails in a greater degree than 

 in any of the higher classes, so we may conclude that this haemal arch pre- 

 sents, by its articulation with the epencephalic neural arch, its normal position; 

 and that the whole occipital vertebra here manifests its veritable and typical 

 character. 



As the haemal arches in the trunk of fishes commonly support diverging 

 appendages, which project freely outwards and backwards, but are hidden and 

 buried in the muscular masses to which they give attachment, so the occipital 

 arch, also, commonly supports its diverging appendages. They are absent 

 in Gymnoihorax and some other Murcenidce. The appendage is present in 

 the form of a single multiarticulate filament in the eel-like protopterus* and 

 lepidosirenf ; it is modified by that mode of vegetative repetition which 

 results in adding to the number of similar filaments directly articulated to 

 the supporting arch ; and is further complicated by the expansion or conflu- 

 ence of the proximal joints in different degrees as they recede from the sup- 

 porting arch, so as to constitute definable segments of the appendagej. 



Such is the condition of the part in most osseous fishes, and such is shown 

 in the diagram of the base of the appendage in figure 5 ; where the proximal 

 segment consists of two broad and flat bones (54 and 55), the next segment of 

 five narrower and shorter but thicker bones (56), and the last segment of 

 more numerous bones of the primitive filamentary form and multiarticulate 

 structure, which bifurcate and radiate as they recede from the centre of at- 

 tachment. 



We may connect the tendency to extreme and variable development in the 

 peripheral parts of a vertebral segment, with the freedom which is the neces- 

 sary consequence of their position : they are attached by one end only, they 

 have not, therefore, that physical restraint to growth which may arise out of 

 the fettering by both extremities, which characterizes the more central ver- 

 tebral elements entering into the composition of the neural and haemal arches. 

 Even in these we find the disposition to luxuriant growth or vegetative sub- 

 division greatest in the peripheral elements, viz. the neural and haemal spines : 

 much more, therefore, might it be expected in the less constant, diverging, 

 and commonly freely projecting appendages of the vertebral arches. Although 

 here the polarizing forces which tend to shoot out particle upon particle after 

 the pattern of dendritic corals, plants or crystals, are so controlled by the 

 antagonizing principle of adaptation, that the radiating growth is always 

 checked at that stage and guided to that form which is suited to the wants 

 and required by the mode of life of the species. 



Since, however, we are able to retain firtnly and with certitude our recog- 

 nition of the special homology of the diverging appendage of the occipital 

 haemal arch, through all its modifications, from the single ray of the lepidosi- 

 ren to the hundred-fold repetition of the same elements with superadded 

 dichotomous bifurcations sustaining the enormous pectoral fins of the 

 broad and flat plagiostomous fishes thence called ' rays ' par excellence, so 

 we can retrace, with equal certitude, the serial homology of this appendage, 

 when it is so plainly manifested by its simple form as well as connections in 



* Linna?an Transactions, vol. xviii. pi. 23, fig. 4, w. 



t Bischoff, Lepidosiren paradoxa, 4to, pi. 2, fig. 4, c. 



% Hunterian Lectures on Vertebrata, figs. 27, 40, 41, 42, 43, 75. 



