1 2 ALLIS. [VOL. I. 







There are thus in the recent state of Amia two morpho- 

 logically distinct openings fused to form the single fenestra of 

 Sagemehl's descriptions of the prepared skull. The same is 

 true of all teleosts that I find described, but the conditions are 

 there less obvious than in Amia. Moreover, in those teleosts 

 in which the interorbital part of the skull is reduced to an 

 impair interorbital septum, another opening is, or may be, added 

 to the two optic fenestrae of Amia ; the orbital region of the 

 prepared skull of such fishes presenting three openings, two of 

 which are lateral and one median. This is evident in Sage- 

 mehl's descriptions of Macrodon (No. 27, p. 67), and in the 

 sectional view given by him of the orbital part of the skull of 

 Erythrinus (No. 27, PI. I, Fig. 8). The two lateral openings 

 lead from orbit to orbit, and include, in their anterior portions, 

 those perforations of the interorbital wall or septum that are 

 said to be formed around the optic foramina. In their posterior 

 portions they contain the orbital openings of the eye-muscle 

 canal. The median opening lies between the postero-superior 

 margins of the lateral openings and leads directly from the 

 orbits into the cranial cavity. The flatter the hind wall of the 

 orbit, and the larger the interorbital perforation, the more sep- 

 arate and distinct does the median opening of the brain case 

 become, as Brooks' side view of the skull of the haddock 

 plainly indicates (No. 8, PL V, Fig. i). The lateral openings 

 are the optic fenestrae as defined by Sagemehl ; the median 

 opening, wrongly called by him the optic fenestra in Macrodon, 

 may be called the orbital opening of the brain case, or simply 

 the orbital fontanelle. The optic fenestrae of Sagemehl are 

 apparently the orbito-sphenoiclal fenestrae of Parker's descrip- 

 tions of Lepidosteus (No. 21, p. 480). 



The optic fenestrae of teleosts, as above defined, are, in the 

 fresh skull, entirely closed by membrane, excepting in their 

 ventro-posterior portion, where the membrane on each side of 

 the head is interrupted by the orbital opening of the eye-muscle 

 canal. The anterior or antero-ventral part of the membrane of 

 each side is, in those fishes in which a median, interorbital sep- 

 tum is found, fused with the corresponding part of the membrane 

 of the opposite side to form that septum. The dorso-posterior 



