778 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [32] 



Amia; there are also nerves and muscles to be found in it. The facial, 

 with its ramus palatinus, and the trigeminus course through the postero- 

 lateral divisions of this space, as already stated, between the mem- 

 brane and the bony lateral wall of the skull for some distance before 

 they arrive at their foramina of exit. In the anterior part of this space 

 the membrane is perforated by the opticus. 



In the lower part of this cavity, which is separated as we have de- 

 scribed from the brain case, are to be found the points of origin of the 

 external rectus muscle. These arise near each other not far from the 

 median line, close behind the cartilaginous transverse bar, already re- 

 ferred to above, that forms the anterior boundary of the sella turcica; 

 anteriorly these muscles diverge from each other, each to enter an orbit 

 through the optic foramen on either side. So we find in Amia, as in so 

 many of the bony fishes, a subcranial canal, which to be sure is but 

 feebly defined, lacking as it does a superior osseous partition to divide 

 it from the cranial cavity. The nervus abducens perforates the fascia 

 from above, and immediately passes into the substance of the external 

 rectus muscle, so that it is not visible in the orbit proper. In addition 

 to this, the principal branches of the carotid artery are to be found in 

 this subcranial canal. Upon the membrane above this canal lie the 

 hypophysis cerebri and the lobus vasculosus in a feebly developed funnel- 

 shaped depression. 



We will now turn our attention again to the two ossifications, found 

 in the lateral angles of the anterior cartilaginous bar. These cannot be 

 observed from the outside, and it is only in the dissected skull and after 

 the fascia has been removed, that they are exposed to view. Bridge 

 has called these parial ossifications the basisphenoidea and declares that 

 they are homologous with the well-known T-shaped basisphenoid of 

 many of the osseous fishes. 



This statement I fully indorse. If we bear in mind that besides the 

 recti externi, the other muscles of the eye also make their appearance in 

 the cavum cranii, then the cartilaginous partition lying between these 

 two groups of muscles must necessarily be implicated, and the two cen- 

 ters of ossification already spoken of must through extension eventually 

 meet and merge into each other, forming a non-j)arial bone, situated 

 between the muscles of the right and left bulbus. It is then that we 

 have the conditions presented to us seen in so many of the bony fishes. 



If this explanation be not accepted, then we must see in Ainia certain 

 ossifications that occur in no other fish, and must deny Amia a bone of 

 very frequent occurrence. 



The next thing before us is to compare the subcranial canal, which 

 lodges the muscles of the eye in Amia with that canal as found in osse- 

 ous fishes, and endeavor to ascertain whether it cannot be traced to a 

 known and similar structure in forms occupying a lower position in the 

 scale. I will first briefly compare it with the canal as found in the 

 Teleostei. 



