OF THE EARLIER VERTEBRATA. 43 



In the heads of the osseous fishes, the cranium proper, 

 though consisting, like the skulls of birds, reptiles, and mam- 

 mals, of several bones, exists from snout to nape, and from 

 mastoid to mastoid, as one unbroken box ; whereas all the 

 other bones of the head, such as the maxillaries and inter- 

 maxillaries, the lower jaws, the opercular appendages, the 

 branchial arches, and the branch iostegous rays, are connected 

 but by muscle and ligament, and fall apart under the putre- 

 factive influences, or in the process of boiling. This unbroken 

 box, which consists, in the cod, of twenty-five bones, is the 

 liomologue of that cranial box of the placoids which consists 

 of one entire piece, and the homotype^ according to Oken, 

 of the bodies and spinal processes of four vertebrae ; while 

 the looser bones, which drop away, represent their ribs. The 

 upper surface of the box, — that extending from the nasal bone 

 to the nape, — is the only part over which a dermal buckler 

 could be laid, as it is the only part with which the external 

 skin comes in contact ; and so it is between this upper sur- 

 face and the cranial bucklers of the early ganoids that we 

 have to institute comparisons. For it is a curious fact, that, 

 with the exception of the Old Ked genera Acanthodus, Gheir- 

 acanthus^ and Diplacanthus,* all the ganoids of the period in 

 which ganoids first appear have dermal bucklers placed right 

 over their true skulls, and that these, though as united in their 

 parts as the bones proper to the cranium in quadrupeds and 

 fishes, are composed of several pieces, furnished each with its 

 independent centre of ossification. The Dipterians, the Ce- 

 lacanths, the Cephalaspians, and at least one genus placed 

 rather doubtfully among the Acanths, — the genus Cheirolejns^ 

 — all possessed cranial bucklers, extending from the nape io 

 the snout, in which the plates, various, in the several genera, 

 in form and position, were fast soldered together, though in 

 every instance the lines of suture were distinctly marked. 



* Tho Acanths of the Coal Measures possess the cranial buckler. 



