80 ILLINOIS BIOLOGICAL MONOGRAPHS [80 



a thin lamella of bone formed the margin, as if the cartilaginous connexion 

 between the parachordal and otic capsule had been stretched, until only the ossi- 

 fied perichondrium remained between them (Fig. 4). The other thin part of 

 the bone lies near its posterior margin, just dorso-posterior to its suture with 

 the basioccipital (Fig. 20). This part was formed from the outer lamella alone, 

 as is stated in the discussion of the 32 mm. stage, and was sohd cartilage in the 

 10 mm. stage. 



The external surface of the bone is smooth, except for a shallow depression 

 near the dorsal margin where the adductor hyomandibularis is inserted (Fig. 20). 

 The bone is almost square in outline and has anterior, dorsal, and ventral edges 

 serrated, where they overlap the other bones. The posterior margin is straight 

 and is separated from the exoccipital by a thin strip of cartilage. Antero- 

 dorsally it interdigitates with the sphenotic below the hyomandibular facet, 

 but takes no part in the formation of the latter. Dorso-posteriorly this line 

 of interdigitation continues between the prootic and the anterior end of the 

 pterotic. Ventro-anteriorly it is overlapped by the parasphenoid and behind 

 by the basioccipital. No foramina are present in the body of the bone for the 

 passage of nerves or blood vessels as these all leave by the trigemino-facial 

 foramen. 



The internal surface of the bone is not as regular in outline nor does it pre- 

 sent the same smootlmess as the outer (Fig. 7). As remarked above, the bones 

 of the two sides are fused in the middle line and their anterior edges are over- 

 lapped by the posterior margin of the suprasphenoid bone which extends from 

 the ventral margin of one trigemino-facial foramen across the floor of the cra- 

 nium to the other. There is a shallow depression in the floor of the cranimn just 

 behind this contact with the suprasphenoid, the sella turcica, for the 

 hypophysis. The floor of the sella is very thin, but immediately posterior to it 

 there is a massive ridge, the dorsum sellae, which is continuous posteriorly with 

 the bulk of the basioccipital bone. This is not completely ossified, but consider- 

 able cartilage, continuous with that within the basioccipital, remains between 

 the inner and outer lamellae. 



The median section shows the relations of the prootics to the parasphenoid 

 and basioccipital in this posterior region (Fig. 7). The recessus sacculorum 

 project anteriorly into the posterior ends of the prootics and medial to them the 

 transverse suture between the inner lamella of the prootics and the basioccipi- 

 tal is visible. In the 32 mm. stage (Fig. 27) a thin horizontal lamella from the 

 median cartilage extends laterally above the anterior end of each recessus to the 

 ventral edge of the otic capsule, but no lamella was yet formed on the floor of 

 the recessus. In the adult, this basal lamella is present and, together with a 

 lamella of the basioccipital, extends ventrally into the recess. The prootic 

 lamella forms both floor and side walls of the recessus and is fused with the 

 ventral face of the horizontal lamella, but the basioccipital lamella meets a 

 descending lamella of the exoccipital half way up in the lateral wall. Thus in 



