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appearance in that part of the cartilaginous wall of the cranium 

 which immediately surrounds the auditory labyrinth. One of these 

 is anterior, and becomes the petrosal ; one is posterior, and even- 

 tually unites with the exoccipital ; the third is superior, and in the 

 end coalesces with the supraoccipital. The posterior ossification 

 clearly represents the mastoid, and it is most interesting to find it, 

 in this early condition, as distinct as in the Chelonian. 



The superior ossification has only to increase in size and remain 

 distinct in the same way as the mastoid of the turtle remains di- 

 stinct, to occupy the precise position of the "occipital externe" of 

 the fish. But, further, it is most important to remark, that when 

 this primarily distinct bone has coalesced with the supraoccipital, 

 it stands in just the same relation to that bone, to the petrosal, to 

 the mastoid and to the semicircular canals, in the snake, as that 

 lateral element, early confluent or connate with the supraoccipital 

 in the chick, which I have termed the " os epioticum." I believe, 

 then, that this "os epioticum," distinct in the young snake, but 

 afterwards confluent with the supraoccipital, and becoming what may 

 be termed the epiotic ala of that bone in the adult, is the homologue 

 of the corresponding bone, or confluent ala of the supraoccipital, in 

 birds and reptiles, while in the fish it remains distinct, and constitutes 

 the " occipital externe." 



For the rest, the superior part of the cranial arch in the carp re- 

 sembles that of the bird. There are a supraoccipital, two parietals, 

 and two frontals ; the squamosal occupies the same position as in 

 the chick, and as in the latter, is, in the dry skull, visible from 

 within, in front of the petrosal. 



As in the Amphibia, both the mandibular and the hyoidean arches 

 are suspended by a pedicle or suspensorium, which is, to a certain ex- 

 tent, common to both, and presents a complexity of structure which 

 can only be elucidated by the most careful study of development. 



In ordinary fishes, such as the carp, stickleback, &c., the proximal 

 end of the suspensorium is constituted by a single bone, Cuvier's 

 " temporal" whose cranial end abuts against the squamosal, petro- 

 sal, and post-frontal bones. 



This temporal* gives off posteriorly a process to which the 



* In adopting the universally known Cuvierian appellations, I merely desire to 

 avoid for the present all theoretical suggestions. 



