770 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [24 | 



through the entire thickness of the skull wall, and therefore remain sep- 

 arated from the cranial cavity by a layer of cartilage. 



The exoccipital, the intercalare, and the post- and prefrontal are found 

 to be in this condition. 



It is hardly worth while mentioning that the squamosal also belongs 

 to this category, applied as it is, in most fishes, to bound a portion of 

 the outer arch ; a like condition obtains in Amia, where, as has been 

 fully discussed, it retains the character of a cover-bone throughout 

 life. 



Within the cavum cranii the anterior part of the occipital region is 

 very sharply defined by an elevation directed anteriorly and towards the 

 median line, composed partly of cartilage and partly of membrane, 

 which runs along the lateral wall from above downwards, forming the i 

 posterior wall of the niche of the labyrinth. The base of this region is i 

 formed by the basioccipital, by the lateral walls, and for the greater ; 

 part also by the cover-bone of the occipitalia lateralia; the adjoining , 

 portion of the spinal canal, which is covered by the occipital arch pos- 

 teriorly, does not lie in the same plane with the base of the brain cavity, 

 but is found higher up on the i30sterior wall of the skull, so there re- • 

 mains a fossa in this locality, which terminates blindly behind and 

 below, over which the medulla oblongata and the anterior end of the 

 spinal marrow pass. This depression is filled in with the now recog- 

 nized interdural lymphatic fat tissue, ^-^ most extensively found in the i 

 Teleosteans, and becomes interesting to us for the reason that in the 

 family of Oharacinides, Cyprinoides, the Shads andGymnotides, it is this ■ 

 very depression that is partitioned off from the rest of the skull cavity , 

 by the crests of the occipitalia lateralia, which meet mesially, and is ■ 

 utilized for the formation of the "atrium sinus imparls," which is con- 

 nected with the swim-bladder by means of the apparatus of Weber. 

 The broad foramen for the vagus is situated at the anterior margin of 

 the occipitale laterale. The anterior border of the labyrinth region 

 within the brain case is formed by the anterior margin of the petrosal 

 which does not join with the anterior bounding ledge of the labyrinth 

 niche, but runs a little before it. The exceedingly complicated struct- 

 ure of the labyrinth niche, with the canals for the arches, is for the 

 most part cartilaginous ; its lateral wall is only formed by the petrosal 

 below and anteriorly. The labyrinth is divided by a medial and pro- 

 jecting cartilaginous elevation, running anteroposteriorly and from 

 above downwards into two fossse, the smaller one being situated an- 

 teriorly and above, the larger one posteriorly and below ; the former 

 contains the greater part of the utriculus, the latter is intended for the 

 sacculus with the recessus cochlearis. The recess for the sacculus forms, 

 as I have already had occasion to state, quite a prominence on the 

 lateral wall of the skull, which is to be regarded as the commencement 



^ Usually this fat tissue of fishes is taken for the arachnoid in these forms. I have 

 reserved my full reason for a dissenting view for a later work. 



