41] THE SKULL OF AMIURUS— KINDRED 41 



parachordal away from each other, without a corresponding growth of the 

 immediate connecting region. 



Posterior to the h>'pophysis, the parachordals gradually approach each 

 other and the fenestra between them narrows to a mere slit as compared with 

 the wide fenestra in this region of the younger cranium. The parasphenoid 

 ossification forms the floor of this fenestra and a couple of processes from its 

 dorsal surface project into the cavum cranii (Fig. 4, Ps.). With fusion of the 

 parachordals with each other farther back, the parasphenoid is excluded from 

 the floor of the cavum cranii and lies ventral to the cartilage thus formed 

 (Fig. 27). In the layer of osteoblasts between the periosteum of the para- 

 sphenoid and the cartilage the anterior projection of the perichondrial ossifica- 

 tion on the ventral surface of the posterior part of the basal plate, appears as a 

 couple of spicules. 



At the line where the parachordals (Fig. 4) fuse with each other, they also 

 form a cartilaginous continuum with the otic capsules, the perichondrium of 

 which is ossified. Laterally and above the parachordal-otic capsule fusion, the 

 cartilage of the capsular wall, which was thick in the younger stage, is reduced 

 to an osseous plate continuous with the perichondrium of the cartilage above 

 and below it (Fig. 4). The edges of this thin region are abrupt and the macula 

 utriculi abut against the wall, giving evidence that the thinness of the wall 

 was caused by the lateral growth of the utriculus. 



There is a sm.all recess in the floor of the cranium on either side in the region 

 where otic capsule and parachordal fuse. Each recess is covered dorsally with 

 a thin lamella of bone and osseous trabeculae (Fig. 27) extend across its lumen. 

 Proceeding posteriorly, these recesses converge and the anterior ends of the 

 sacculi appear within them (Fig. 27). These recesses were not present in the 

 younger stage, but have been caused by the anterior growth of the sacculi 

 beyond the canal sinus imparls, and subsequent growth of the cartilage of the 

 floor around them. The osseous roof above these recesses is apparently 

 formed by the ossification of perichondrial strands from this cartilage (Fig. 

 27). These recesses contain the anterior ends of the sacculi ('processes of 

 Comparetti,' Wright, 1884) and are said to lie within the prootic bones of the 

 adult. For a short distance posteriorly the 'processes of Comparetti' are 

 separated from each other by a median crest of cartilage (Fig. 28), the dorsal 

 surface of which is grooved and filled with osseous trabeculae of perichondrial 

 origin. The sacculi grow larger as they approach the canal sinus imparls 

 (Fig. 28), the median cartilage diminishes in amount and the two recesses come 

 closer together. The osseous roof of each recessus disappears in this region 

 where the ramus saccularis of the auditory nerve descends to innervate the 

 macula sacculi (Fig. 28). Just posterior to this innervation the sacculi of the 

 two sides are connected with each other by the canal sinus imparls. 



In the other teleosts whose developmental history has been followed, the 

 ossifications which arise in this region around the anterior parachordals and 



