43] THE SKULL OF AMIURUS—KINDRED 43 



esses connected by sutures to the descending processes of the roof plates of the 

 recessus sacculorum. Thus there are formed three chambers; the two lateral 

 contain the sacculi and the lagenae, while the medial unpaired chamber con- 

 tains the sinus impar of the Weberian apparatus, and is called the cavum sinus 

 imparls. The floor of each recessus sacculi is formed by a thin osseous lamella, 

 as the cartilage which was originally in this region has been resorbed and the 

 perichondrial ossification alone remains. The ventral surface of the median 

 basal plate is covered by a thick perichondrial ossification which extends 

 laterally beneath the recessus sacculorum and with which the parasphenoid 

 ossification interdigitates. In cross-section, the notochord appears as a very 

 small circular area within the median basal plate, the tip lying just beneath 

 the region of the canal sinus imparls. In the younger stage it extended anterior 

 to this connexion and in this immediate region was very large in com- 

 parison with the amount of the surrounding cartilage. The plate at that stage 

 (10 mm.) was divided into halves (Fig. 8), but now forms a continuum above 

 and below the notochordal tip (Fig. 27). 



The amount of cartilage around the notochord gradually diminishes toward 

 the posterior end of the cranium, while the diameter of the notochord increases 

 (Fig. 26). The whole floor of this region has become deeper and narrower than 

 it is anteriorly and the ossification in the floor of the recessus sacculorum has 

 replaced the cartilage entirely. The former cartilage between the glosso- 

 pharyngeal and vagus nerves, has been resorbed and the lamella formed by the 

 ossification of its outer perichondrium alone is left, separating the two nerves 

 (Fig. 4). The vagus nerve issues posterior to this lamella through a very 

 elongate foramen (x) between the otic capsule and the dorso-lateral edge of 

 the recessus sacculi of that side. Posterior to this foramen the perichondrial 

 ossification at the ventral end of the occipital arch is continuous with that on 

 the roof and side walls of the recessus sacculi. This lamella is, however, only 

 an anterior projection of the perichondrial lam.ella around the anterior surface 

 of that part of the occipital arch which joins the cartilage of the roof of the 

 recessus more posteriorly (Fig. 23). This cartilage is not very great in extent 

 (Fig. 23) and osseous trabeculae again appear between the saccular recesses 

 and the posterior ventral end of the occipital arch. The hypoglossus nerve, 

 which in the younger stage issued from the cranium through a rather large 

 space, is now enclosed within this osseous mass {ex). 



Solid cartilaginous masses on each side of the notochord mark the posterior 

 ends of the recessus sacculorum, just posterior to the passage of the hypoglossus 

 ipch) . These cartilaginous masses extend farther posterior than in the younger 

 stage and rise higher on the sides of the notochord (compare Figs. 17, 23) 

 so that there is a groove between them filled with osseous tissue (Fig. 37) 

 The whole notochord is surrounded with osseous tissue and there is an hypo- 

 chordal cartilage present on its ventral medial surface. This piece of cartilage 

 was not developed in the 10 mm. stage. Anterior to the hypochordal cartilage 



