38 ILLINOIS BIOLOGICAL MONOGRAPHS [38 



from the alisphenoid cartilage (Fig. 4), forms the wall between the optic and 

 trigeminal nerves. 



Ventral to the trigeminal nerve, the cartilage reappears within the ossifica- 

 tion, but unconnected with it as before. This cartilage becomes larger pos- 

 teriorly, until, posterior to the passage of the facialis nerve, ossification has 

 again become limited to the median ventral surface and has only a couple of 

 splints protruding into the cavum cranii (Fig. 3) through the anterior fenestra 

 basicranii. 



The question now arises as to the proper designation for this part of the 

 basal fenestra, which lies behind the hypophysis, in the salmon (Parker, 1872; 

 Gaupp, 1906) it is termed the fenestra basicranii anterius, in contradistinction 

 to a more posterior parachordal fenestra. The cartilaginous plates on either 

 side of the cranial floor in this region of Amiurus have grown anteriorly in 

 concert with the otic capsules, since their lateral ends are confluent with the 

 perichondrial ossification of the ventral side of the otic capsules (Fig. 4). The 

 small medial space between them is a fenestra between the anterior para- 

 chordals and is therefore comparable to the corresponding fenestra in Salmo 

 and may be so designated. It is evident therefore, that the connexion between 

 trabeculae and parachordals has disappeared by resorption of cartilage. The 

 dorsal ossification on the parasphenoid, the suprasphenoid, has taken its place. 



In Gasterosteus (Swinnerton, 1902) the trabecular cartilage has disap- 

 peared in this region as in Amiurus, but the floor of the cranium is formed by 

 the parasphenoid, except in the region of the eye-muscle canal ; here there is an 

 ossification corresponding to the suprasphenoid of Amiurus. Unfortunately 

 the details of the development of this region is not known. The modifications 

 of the basal part of the cranium, in forms having an interorbital septum, where 

 the parts are all compressed, are not easily homologized with the depressed 

 condition of the same region in Amiurus, and in making comparisons, this fact 

 always has to be borne in mind. Also the absence of an eye-muscle canal in 

 Amiurus makes for differences in the development of the basal parts between 

 the orbits. 



In Salmo there is an elongate trabecula communis (Parker, 1873; Winslow, 

 1897; Gaupp, 1906) which is concave below and bears the membranous inter- 

 orbital septum on its crested dorsal surface. I have found no mention of a 

 perichondrial ossification around this trabecula communis, such as is found 

 at the anterior ends of the trabeculae in Amiurus between the orbits, and which 

 is the anlage of the orbitosphenoid of the adult. The parasphenoid has the 

 typical relation to the floor of the cranium that it usually has in the Ich- 

 thyopsida. 



McMurrich (1884b) has described a basisphenoid in this region of the adult 

 Amiurus, as an "indistinct ossification completely fused with the parasphe- 

 noid. " I have remarked above that there is an ossification on the dorsal 

 surface of the parasphenoid between the trabeculae and have it named the supra- 



