84 ALUS. [Vol. XVIII. 



Where the petrosal is overlapped by the intercalar the extreme 

 outer edge of the latter bone projects, in places, under short scale- 

 like processes on the outer surface of the petrosal. Similar scale- 

 like processes of the occipitale laterale overlap, also, the outer 

 edge of that part of the intercalar that lies superficial to it. While 

 all of these processes are excedingly delicate, and unimportant 

 in size, they are always found, and hence may possibly represent 

 a stage in that primary assimilation of the intercalar that Sagemehl 

 believed in. 



The mesial, horizontal process of the petrosal has, in Scomber, 

 two portions, a posterior and an anterior one. The posterior 

 portion lies posterior to the pituitary opening of the cranial cavity, 

 the anterior portion anterior to that opening. The posterior por- 

 tion thus corresponds to, and is the homologue of, the entire 

 horizontal process of the petrosals of Aviia and the Characinidse, 

 in all of which fishes the anterior edge of the process lies posterior 

 to the pituitary body (No. 64, p. 214, and No. 65, p. 87). In 

 most of the Cyprinidse the same conditions and relations are found, 

 but in certain of those fishes (No. 66, p. 575) an arrangement 

 somewhat similar to that found in Scomber seems to exist. The 

 descriptions of these latter fishes are, however, not concise in this 

 particular, and what is bone in Scomber may, in the cyprinoids 

 referred to, be simply membrane. 



The posterior part of the process, in Scomber, adjoins poster- 

 iorly the anterior edge of the basioccipital, and medianly the 

 corresponding edge of its fellow of the opposite side of the head. 

 It is separated from the basioccipital by a narrow line of cartilage, 

 which, in the middle line of the head, extends forward between the 

 adjoining edges of the processes of the two sides of the head for 

 about one half their length. The process forms, in Scomber, as 

 in other fishes, a part of the floor of the cranial cavity, and a part 

 of the roof of the eye-muscle canal. It is pierced, near its lateral 

 edge, and somewhat in front of the middle of its length, by the 

 foramen for the nervus abducens {abfr, Fig. 22). Its anterior 

 edge forms the posterior edge of the pituitary opening, that open- 

 ing occupying only about two thirds the width of the process {pto, 

 Fig. 22). Lateral to the pituitary opening a narrow part of the 

 process connects its posterior portion with the anterior portion, 



