219 



2. Skull. 



In some instances even the sheath of the cranial part of the chorda 

 ossifies in its hindermost part, and forms a true vertebral body 

 for the occipital vertebra t which entirely corresponds to those of 

 the column. This has been observed by me as yet in Lepto- 

 cephalus and several Squalidse ; but it is extremely probable that 

 the basilar occipital of all osseous fishes, viz. that part of this 

 bone which resembles a common vertebral body, is developed quite 

 in the same way. 



C. On the manner in which the outer ossifying layer is concerned 

 in the formation of the bodies of the vertebrae. 



1st. In those cases where the outer ossifying layer, viz. that layer 

 in which the cartilaginous arches are developed, takes part in the 

 formation of the vertebral bodies, there are to be distinguished two 

 different processes, one in which the crural cartilages themselves 

 play a part in this formation, and a second, where only the periosteal 

 layer between them is concerned. 



2nd. Where the crural cartilages take a part, they form, in the 

 first place, by their coalescence an outer cartilaginous layer around 

 the body of the vertebra, which took its origin from the chorda, and 

 which we shall henceforth call the chordal vertebral body. 



3rd. This outer cartilaginous layer ossifies in many cases; and 

 this ossification may take place in two places only, viz. on the right 

 and left side of the vertebral body, as in Heptanchus, or in four 

 places, in which case a superior point of ossification at the floor of 

 the neural canal, and an inferior one at the roof of the haemal canal, 

 are added to the two lateral ones (Acanthias, Scymnus}. 



4th. These external ossifications of chondriform bone may retain 

 their primitive form of plates, and may then be called the lateral, 

 superior, and inferior osseous plates ; or they acquire by additional 

 growth, at the expense of the outer cartilaginous layer, the form of 

 wedge-shaped or cuneiform bodies, and may be named the lateral, 

 superior, and inferior wedges (Zapfen, Keile, Germ.). 



5th. In both cases these external ossifications comport themselves 

 in two different ways with regard to the chordal vertebral body, 

 inasmuch as in some cases both coalesce at their ends (Scymnus, 

 Acanthias)) whilst in others they remain separated (Heptanchus) . 



