ON THE VERTEBRATE SKELETON. 279 



The correspondence with the scapular, or occipito-haemal arch, is further 

 carried out by the presence of appendages (44) which freely diverge from it, but 

 the development of these appendages has not been observed to extend beyond 

 that second phase, marked by vegetative multiplication of the simple ray, 

 directly attached to the arch itself. The lepidosiren offers the simplest con- 

 dition of such ' diverging appendage ' in the single slender bony piece con- 

 nected with the element 40*. Cuvier and other ichthyologists cite a series 

 of stages of this kind of development of the hyoidean appendage from a si- 

 milar simple beginning up to a 30-fold repetition of the single ray (Elops); 

 and the ' branchiostegal ' rays have been found in much greater numbers in 

 certain fossil fishes. Like the ' pectoral ' rays, they support a duplicature of 

 membrane, which plays freely backwards and forwards, reacting upon the 

 ambient medium, and forming, in short, a cephalic fin, but with its powers 

 so restricted and adjusted, as to propel the water through the branchial cham- 

 bers of the fish, instead of driving the fish through the water ; in which latter 

 action, indeed, the occipital appendages (pectoral fins)in most osseous fishes 

 can and do perform but a very small share. 



If we next proceed to compare the frontal segment, N m and H in, dis- 

 membered as above described from the parietal vertebra, and, by the separa- 

 tion of the sutures, from the bones terminating the skull anteriorly, we shall 

 find a neural arch (fig. 3) closely repeating the characters of that of the oc- 

 cipital vertebra. The centrum is sometimes represented simply by the forward 

 extension of ossification of the basisphenoid (11), which I regard as the ho- 

 motype of the ossification of the capsule of the notochord beneath the cen- 

 trums of the anterior trunk-vertebras in the silurus ; sometimes, also, of a di- 

 stinct superincumbent symmetrical ossicle (9', fig. 5), answering to the rudi- 

 mental (central part of the) body of the atlas supported by the inferior bony 

 plate, inthesilurus. This more complex condition of the centrum of the frontal 

 vertebra is well-seen in the sword-fish. The bones 10, 10, which directly rest 

 upon 9', when it exists, which defend the sides of the prosencephalon, and 

 which are either grooved by the optic nerves, or have those nerves perforating 

 the fibro-cartilaginous membrane close to the margin of the bone (10) from 

 which it is continued, are obviously the neurapophyses. They are, however, 

 small ; inasmuch as the segment of the brain to which they relate is of inferior 

 size in bony fishes : and they are still smaller in comparison with the spine 

 (11) which is enormously expanded, in relation to its accessory functions as 

 the chief contributor to and protector of the orbits. The bones 12, wedged 

 between the neurapophyses and spine, affording an articular surface to the 

 proximal piece of the haemal arch, and developing a transverse process for 

 muscular attachments, are the parapophyses. The bones (17) have as little 

 essential connection with the typical neural arch above demonstrated, as the 

 bones 16, 16" had with the corresponding arch of the parietal vertebra : and 

 their more peculiar form in relation to the ball which they protect, and their 

 variable histological condition in the vertebrate series, have not only prevented 

 their ever being mistaken for parts of cranial vertebrae, but have led to the 

 opposite extreme of excluding them altogether from the bones of the skull, 

 with which they are as much entitled to rank as the petrosal (\q) or the 

 turbinal (19) ; but always in the category of sense-capsules or ' splanchno- 

 skeletal ' pieces. 



In regard to the inferior arch of the frontal segment, the subdivision of its 

 constituent elements, in subserviency to its special functions, is carried to as 

 great an extent as in that of the parietal segment. I regard the four over- 

 lapping and closely-connected pieces from the upper joint (asa) to the lower 

 * Hunterian Lectures on Vertebrata. p. 79, fig. 27, 37. 



