37] THE SKULL OF AM lURUS— KINDRED 37 



because the centres of the frontals appear before there is any trace of the lateral 

 osseous canals, and that, in some forms, the osseous canals are entirely separate 

 from the frontals throughout life, but in the majority of forms they fuse with 

 the underlying independent frontal. The frontals of Amiurus bear out this 

 latter statement, since the rudiments appear as paired membranous sheets 

 roofing the large fontanelles. The lateral line canal ossification arises 

 independently of this membrane and only secondarily becomes connected 

 with it. This independence of ossifications shows especially well in the 

 relation of the ossification surrounding that canal extending from the jimction 

 of the suborbital and supraorbital canals toward the middle line of the head; 

 here the canal ossification is very distinct from the underlying frontal. At this 

 stage in Amiurus the frontals have practically the same relations to the sur- 

 rounding bones that they have in the adult cranium. 



The infraorbital chain of bones, whenever developed, is related to the sub- 

 orbital lateral line canal, and in most cases the component bones of the chain 

 are larger and flatter than in Amiurus, a condition usually correlated with 

 the development of the eye. 



The anterior ends of the trabeculae are as yet cohtinuous with the posterior 

 end of the ethmoid plate, although the fenestra hypophyseos is more posterior 

 than it was in the younger stage (Fig. 3). They are no longer continuous bars 

 from the ethmoid plate to the parachordal plate, but half way between these 

 regions a part of each has been resorbed and parasphenoid and suprasphenoid 

 ossifications have replaced it by growth dorsally into this region, forming the 

 posterior margin of the optic foramen and part of the margin of the trigemino- 

 facial foramen (Fig. 4). 



The trabeculae of the two sides are connected across the anterior end of 

 the fenestra hypophyseos by a perichondrial ossification, separated by a wide 

 space from the more ventraUy situated parasphenoid ossification (Fig. 4). 

 This ossification lies in the floor of the cranium between the optic foramina. 

 Anterior to these foramina the trabeculae are united to the anterior ends of 

 the alisphenoid cartilages to form the cranial wall between the orbital and optic 

 foramina as in the 10 mm. stage. Toward the posterior end of the optic fora- 

 mina, the perichondrial connexion between the trabeculae disappears, and 

 osseous trabeculae, the centre of the suprasphenoid bone, extend dorsally from 

 the parasphenoid ossification, which now forms the medial cavum floor (Fig. 

 32). Farther posteriorly, the suprasphenoid ossification extends above the 

 trabeculae and meets the lateral margins of the parasphenoid external to them, 

 thus encasing the cartilage in an osseous sheath, imconnected with the peri- 

 chondrium. In this immediate region, just back of the preceding part, be- 

 tween the optic and trigeminal nerves, the cartilage of the trabecula disappears 

 (Fig. 3), and the suprasphenoid ossification is connected with the parasphenoid 

 by osseous trabecula across the space formerly occupied by the cartilage. The 

 ossification extends dorsally in the cranial wall and, with a ventral ossified spur 



