33] THE SKULL OF AM lURUS— KINDRED 33 



One of these portions is a thin dense layer of superficial bone. The other por- 

 tion is a deeper one, of quite different appearance, which underlies the central 

 portion only of the superficial portion, and there replaces portions of the carti- 

 lage of the skull. . . . The superficial portion of the bone is represented by 

 a thin plate that lies closely upon the cartilage of the skull, without intervening 

 membrane, and must primarily be wholly of perichondrial origin; But this 

 perichondrial plate receives at certain places, accretions or additions to its 

 outer surface, and these accretions, although they present in sections exactly 

 the same appearance as the perichondrial plate, seem to be of purely perichon- 

 drial origin. This is particularly noticeable, in my specimens, along the lines 

 of articulation of the mesethmoid with the frontals, and in the mesethmoid 

 process. " 



This is identical with the condition in Amiurus. The ectethmoidal rela- 

 tions are also practically identical in the two forms. They are both of peri- 

 chondrial origin with a dermal ossification in the form of a large wing of bone 

 added to them. The other relations of the ethmoidal bones are best discussed 

 in connexion with the adult cranium. 



The orbital region. The orbital wall of the cranium at this stage is formed 

 by the persisting and enlarged alisphenoid cartilage (Figs. 3, 4), together with 

 the ossifications above and below it. It extends from the posterior face of the 

 ectethmoid process as far posteriorly as the anterior end of the otic capsule. 

 The cartilage in this region has grown considerably since the 10 mm. stage, but 

 it has approximately the same topographical relation to the otic capsule and 

 to the optic, trigeminal and facial nerves. The dorsal margins for the fora- 

 mina of these nerves is as yet formed by the unossified cartilage. 



Below the ectethmoid process the cartilage of each wall fuses with the an- 

 terior end of the trabecula cranii of that side and, as in the younger stage, forms 

 the posterior margin of the orbital foramen, separating it from the optic fora- 

 men (Fig. 4). The posterior margin of the latter foramen has developed from 

 the ossification of the membrane which, in the 10 mm. larva, extended from the 

 alisphenoid cartilage to the trabecula. This ossification (Fig. 32) is not a 

 continuum, bat is divided into a dorsal and ventral part. The dorsal part is 

 continuous with the perichondrium of the alisphenoid cartilage as was the 

 membrane of the young animal, but the relations of the ventral part are not so 

 simple and will be taken up in the discussion of the trabecular region. This 

 ossified wall is continued as far posteriorly as the trigeminal nerve (Fig. 4) and 

 forms the anterior margin of its foramen; the dorsal margin has been described 

 above as formed by the alisphenoid cartilage proper {alis. c). The posterior 

 margin of this foramen, through which the main branches of the facial nerve 

 also issue, is formed by a perichondrial ossification (Fig. 4) between the ventral 

 wall of the otic capsule and the posterior end of the trabeculae. 



