THE OSTEOLOGY OF AMIURUS CATUS. 281 



with a thin layer of bone extending across and becoming continuous 

 with the outer perichondral layer of the exoccipital. This thin layer 

 forms the floor of the cavity for the sacculus, and contains no cartil- 

 age, so that the basioccipital at this stage is destitute of cartilage, 

 and is composed of membrane bone in this (anterior) region. More 

 posteriorly, however, behind the exit of the vagus and behind the 

 cavity for the sacculus, the cartilage, continuous with that of the 

 exoccipital, comes down towards the middle line as far as the chorda, 

 which is still surrounded by bone. In an older stage (about 38 mm. 

 in length) the cartilage present around the chorda and on the floor 

 of the cavity for the sacculus is very noticeable. Opposite the exit of 

 the glossopharyngeal, where no cartilage was to be seen in the 

 younger stage, a large plate of it is present at floor of the sacculus- 

 fossa, bearing upon its upper (inner) surface a mass of trabecular 

 bone representing the ossification around the notochord in the 

 younger stage. So opposite the foramen for the vagus (where no 

 cartilage is present in the younger stage) the chorda has much 

 diminished in size, and cartilage is to be seen at its sides below, 

 separated from it by a layer of bone. Still more posteriorly the 

 cartilage has the same relations as in the younger stage. 



It is thus seen that the older stage presents cartilage where in the 

 younger stage only bone is present, apparently reversing the fact 

 that the older the form the less the amount of cartilage present. 

 How is this to be explained 1 In the young stage the sacculus occu- 

 pies the place of the cartilage, being so large in comparison to the 

 size of the skull that there is room only for a thin layer of bone at 

 the floor of the fossa, and a thin investment round the chorda. 

 Later, however, the cranium grows more rapidly than the auditory 

 apparatus, and then the cartilage always present posteriorly grows 

 forward, and, by the ossification of its perichondrium, contributes 

 largely to the formation of the basioccipital. 



The vomer and parasuhenoid are formed in membrane and show 

 no signs of teeth. 



Objections have been made by certain German authors to the 

 application of the terms probtic, epiotic, etc., to the bones developed 

 in the cartilaginous ear-capsule. Vrolik 1 bases his objection to the 

 terms on the fact that other bones, for instance, the supra-, ex- 



1 Vrolik. — Studien uber die Verknocherung u. d. Knochen des Schadels der Teleostei. Nieder- 

 landisches Archiv fiir Zoologie, Bd. I. 1873. 



