768 REPORT OF commissionp:r of fish and fisheries. [22] 



and anteriorly. This missing section is the foramen just described, and 

 its outline depends upon it (Plate I, Figs. 2 and 3, As.). 



i^ear its posterior margin the alisphenoid is perforated by a large cir- 

 cular foramen, intended for the second and third branch of the trigemi- 

 nal. In large specimens of Amia the alisphenoid articulates above and 

 posteriorly with the postfrontal ; in younger individuals it is separated 

 from the latter by a small zone of cartilage. Above the optic foramen, 

 anteriorly, it is to a small extent suturally united with the orbitosphe- 

 noid. 



Beyond the alisphenoid is found the orbitosphenoid, circular in out- 

 line and pierced behind and below for the optic foramen, of which 

 nothing further will be remarked (Plate I, Figs. 2 and 3, Os.). It seems 

 to me that at this point it would not be uninteresting to call attention 

 to the circular form of so many of the ossifications of the primoidal 

 cranium of Amia. 



These forms are due to the fact that the centers of ossification start 

 free in the cartilaginous matrix, and in their unhindered growth, which 

 has been a proportionate increase of margin in all directions, they have 

 but at a few places only been checked by contact with neighboring ossi- 

 fications. In this respect, too, Amia has been preserved in a primitive 

 condition, as compared with the Teleostei, in which the corresponding- 

 bones, owing to the fact of their contact at most points with their neigh- 

 bors, exhibit a great irregularity of form. 



The first branch of the trigeminus passes through the wall of the 

 primoidal cranium at about the height of the anterior margin of the 

 postfrontal, runs obliquely forwards and outwards, and quits the ali- 

 sphenoid just above the large foramen for the second and third branch 

 of the same nerve (Plate II, Fig. 6, tr.^^). 



During its course within the wall of the skull it gives off several mi- 

 nute branches, which ascend upward in the cartilage and pass to the 

 mucus canals of the bones of the skull-cover. In the orbits these 

 branches are two in number, and lie parallel to each other; just be- 

 neath the "cover;" they pass forward to reach the nasal depression to 

 which they are distributed, and in doing so pass between the cartilag- 

 inous cover of the primoidal cranium and the frontal. 



During its entire course through the orbit it gives oft' minute ascend- 

 ing branches, which in part perforate the cartilaginous roof, described 

 above as the remains of the vault of the orbit, which is composed of this 

 material, while another branch passes to be distributed to the mucus 

 canals of the frontal bone. 



The second and third branches of the trigeminus nerve pass from the 

 skull cavity through the foramen in the alisphenoid already referred to, 

 and are distributed in precisely the same manner as they are in the 

 Teleosteans (Plate II, Figs. 4 and .5, Tr.). 



^*Tr. in the figure. — Tkans. 



