326 Royal Society : 



slight traces of ossification, as in Hexanchus and the anterior ver- 

 tebra of Heptanchus, whilst they may be almost wanting in others 

 pretty well ossified (Leptocephalus, Helmichthys, Centrophorus). 



3rd. The ossification of the cartilaginous vertebral bodies formed 

 out of the sheath of the chorda never begins at the surface, but 

 always in their interior, and also in their middle region, and is (as far 

 as I know, without exception) in the first instance a calcified fibro- 

 cartilage, or what I call a fibrous bone. 



4th. The first osseous parts have the form of thin rings (Hep- 

 tanchus, anterior vertebra), which afterwards assume that of hollow 

 and thin double cones (Heptanchus, posterior vertebra, Centrophorus) . 



5th. The growth of these double cones, which are the real osseous 

 vertebral bodies, when once they have assumed their whole length, 

 takes place especially at their outer side, through the addition of 

 calcified cartilage (chondriform bone, Williamson ; Knorpel-Knochen 

 in German), which is formed from the outer chordal cartilage of the 

 vertebral body. In addition to this, the osseous double cone thickens 

 also at the expense of the cartilage inside of it, but in a much 

 smaller degree. 



6th. In some cases the outer growth is everywhere the same, and 

 in this manner stronger double-coned vertebral bodies of uniform 

 thickness are formed. In other cases the growth is in some parts 

 more active than in others, and vertebral bodies then originate with 

 outer ridges and lamellse (Heptanchus > Raia, Carckarias, Mustelus y 

 Galeus). In one single instance the ossification of the outer cartilage 

 takes place in such away that the exterior parts of the vertebral bodies 

 are formed by alternating circles of chondriform bone and cartilage 

 (Squatina). 



7th. With regard to the extension of this growth of the vertebral 

 bodies formed by the ossification of the sheath of the chorda, it is to 

 be remarked that in some cases the whole, or nearly the whole sheath 

 of the chorda ossifies, as in Squatina and the Raiidse. In other cases 

 greater or lesser parts of the primitive cartilage, inside and outside 

 the vertebral body, remain in their primitive state (Squali). 



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, 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 vertebra*. 



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 



