294 ON THE STRUCTURE OF THE VERTEBRATE SKULL. 



body of a vertebra — such as normally each caudal vertebra of a 

 Mammal is ; and that from this, for the purpose of investing the 

 olfactory apparatuses, which are developed at its sides, lamellar 

 processes grow out, which are altogether peculiar to it. In any 

 case, however, the ethmoid may be regarded as the anterior end 

 of the vertebral column. 



" (10.) From what has been stated, it appears that the four 

 different groups of bones — the occipital, with its intercalary 

 bone, the squama ; the basi-sphenoid, with its intercalary bone, 

 the parietals ; the presphenoid, with its intercalary bones, the 

 frontals; and the ethmoid, together with its outgrowths, the 

 spongy bones and the cribriform plate — exhibit in their succes- 

 sive order from behind forwards, a greater and greater deviation 

 from the plan according to which ordinary vertebras are deve- 

 loped, so that the occipital bone is most like a vertebra, while 

 the ethmoid is least like one. 



"(11.) Among the bones of the face, the preinaxillas, the 

 nasal bones, and the vomer are developed altogether indepen- 

 dently of the investing mass of the notochord ; and they never 

 coalesce with parts of the skeleton, which are immediately 

 derived from the latter. On this account, alone, they cannot be 

 regarded as vertebras, or parts of vertebras. Furthermore, they 

 at no time enclose, or help to enclose, a segment of the central 

 nervous system. The nasal bones and the vomer are, properly 

 speaking, ' splint-bones ' (Belegungsknochen) for the ethmoid, such 

 as occur in the vertebras of no animal ; and the premaxillas are 

 applied, although in a different plane, to the one end of the ver- 

 tebral column, as, in Fishes, the median rays of the anal fin are 

 applied to the other end of it.* Furthermore, the palatine bones 

 are developed, together with the pterygoids, in lateral processes, 

 or rays, which have grown out from the middle part of the base 

 of the brain-capsule, and which, as regards their original form, 

 disposition, and connections, resemble the ribs, and may be" 

 regarded as a pair of ribs united with the brain-case. In Mam- 

 malia the two mallei are developed in these two rays, and 



* The study of the development of the skull necessitates the assumption that 

 Sturgeons, Sharks, and Rays have no premaxillpe, and that their skulls end ante- 

 riorly with the ethmoid cartilage. 



