14 ILLINOIS BIOLOGICAL MONOGRAPHS [14 



The fenestra hypophyseos and the fenestra basicranii anterior are closed 

 by a sheet of fibrous connective tissue which stretches between the trabeculae 

 (Fig. 14) and extends anteriorly below the ethmoid plate and posteriorly 

 below the parachordal plate. It is not intimately connected with the cartilage 

 of any of these parts. 



In a 19 mm. larva of Amia, the rectus eye muscles are inserted in a space 

 between the brain and the trabeculae, including some connective tissue with 

 them. In the adult Amia in this region there is a canal separated from the 

 cavum cranii by the prootic ossification. The trabeculae are wider in Amia 

 than they are in Amiurus and form a trabecula communis before fusing with 

 the ethmoid plate. In Amia each internal carotid artery passes through the 

 trabecula on its medial side. The artery gives off a branch above the optic 

 nerve in Amia as it does in Amiurus. The fenestra hypophyseos in Amia is 

 even smaller than in the known higher teleosts. In the cranial wall, anterior 

 to the otic capsule, the fifth and seventh nerves are separated by a bar of 

 cartilage between the trabecula and the otic capsule, as in the Selachians and 

 the Salmonidae, differing in this respect from Amiurus. 



The obHque eye muscles in a 19 mm. Amia are inserted in a foramen in the 

 wall of the cranium between the ectethmoid process and the optic foramen, 

 comparable to the orbital foramen of Amiurus. In Amia this foramen con- 

 tinues anteriorly with a groove on the dorso-lateral surface of the ethmoid 

 plate. Beyond the eye-muscle insertion, the olfactory tractus continues along 

 the anterior part of the same groove. In Amiurus this groove is lacking and 

 the eye muscles do not enter the foramen. There is, however, a concavity on 

 the anterior face of the ectethmoid process which, if continued through to the 

 posterior, would end at the anterior margin of the orbital foramen and may 

 have some significance in comparisons with the anterior continuation of the 

 foramen in Amia. The cartilage of the ethmoid floor in this region between 

 the anterior parts of the orbits, is thicker in Amia than it is in Amiurus. 



In the Acanthias larva (Sewertzoff, 1897), the trabeculae develop as paired 

 independent cartilages at right angles and ventral to the anterior ends of the 

 parachordals, eventually becoming fused with their ventral faces. They 

 grow forward on either side of the hypophysial region of the brain and fuse 

 anteriorly as a trabecula communis plate. As the flexure of the neural parts 

 disappears, the trabeculae become horizontal in position, except in that im- 

 mediate region where they are attached to the parachordal plate. Unlike the 

 trabeculae in Amiurus, the Selachian trabeculae later form a solid floor in the 

 cranium. The cartilaginous connexion between the alisphenoid and trabecular 

 cartilages is far more extensive in the later Acanthias than it ever is in Amiurus. 

 The condition of the cranial floor of Acipenser (Parker, 1882a), is the same as 

 that of Acanthias, although the fenestra hypophyseos may persist for a longer 

 time. In the early Lepidosteus cranium (Parker, 1882b) there is a large 

 fenestra hypophyseos which later becomes closed by the growth medially of 



