ON THE THEORY OF THE VERTEBRATE SKULL SSS 



this bone and that which succeeds it. The inner face of the bone is, 

 as it were, cut short and replaced by this cartilage, whence the inferior 

 edge is also short and is connected only with the basisphenoid, and 

 not with the basioccipital. The anterior margin of the bone corre- 

 sponds with the middle of the mesencephalon, while its inner 

 face presents apertures for the portio dura and portio mollis. The 

 posterior margin of its outer face forms half the circumference of 

 the fenestra ovalis, and it contains the anterior and inferior portions 

 ■of the labyrinth. Thus, with the exception of the absence of an 

 inferior connexion with the basioccipital, — a circumstance fully ex- 

 plained by the persistence in a cartilaginous state of part of the bone, 

 — it corresponds in the closest manner with the petrosal of the bird. 

 I confess I cannot comprehend how those who admit the homology 

 of the bone called petrosal in the bird with that called petrosal in 

 the mammal (as all anatomists do), can deny that the bone in 

 question is also the petrosal, and affirm it to be an alisphenoid. The 

 general adoption of such a view would, I do not hesitate to say, 

 throw the Theory of the Skull into a state of hopeless confusion, 

 •and render a consistent terminology impossible. Where then is the 

 alisphenoid ? I reply, that it is unossified. The posterior portion 

 -of the cartilaginous side-wall of the skull, in fact, unites with the 

 parietal, the petrosal, and the basisphenoid, just in the same way as 

 the bony alisphenoid of the bird unites with those bones. Further- 

 more, as in the bird, it bounds the foramen for the third division 

 of the trigeminal nerve anteriorly, and is specially perforated 

 by the second division of the fifth, while the optic and the other 

 •divisions of the fifth pass out in front of or through its anterior 

 margin. 



Not only is the alisphenoid cartilaginous, but the orbitosphenoid 

 is in the same condition, and a great vertical plate of cartilage 

 represents the whole anterior part of the craniofacial axis, or the 

 presphenoid and ethmovomerine bones.^ It has been imagined, in- 

 ■deed, that the rostrum-like termination of the basisphenoid represents 

 the presphenoid, but I think this comes of studying dry skulls. 

 Those who compare a section of the fresh skull of a turtle with the 

 like section of the skull of a lamb, will hardly fail to admit that the 

 rostrum of the basisphenoid in the turtle is exactly represented by 

 that part of the sheep's basisphenoid, which forms the anterior and 

 inferior boundary of the sella turcica, and that the suture between 

 the basisphenoid and the presphenoid in the sheep corresponds 



' Compare Kolliker's account of the primordial slcull of a young turtle in the ' Bericht 

 ■yon der Konigl. Zool. Anstalt zu Wiirzburg,' 1849. 



