25 



i > iii|Munc jedicle rtirultes by an upper partially divided ball to socket formed by the post- 

 frontal, by the orbito-sphenoid, and by the mastoid. The upper and posterior articular 

 tubercle of the tympanic pedicle supports the operculum. The upper fork of the supra - 

 icapula hat coaletced with the paroccipital : the lower fork articulate* with a transverse pro- 

 ce of the basiorcipiul. 



Presented by Dr. Leach, F.L.S. 



75. The left half of a vertically bisected skull of a large Siluroid fish (Bagrtu tachy- 



pomtu). 



In this specimen the articular surface for the tympanic pedicle is formed by the mastoid 

 and pout frontal exclusively, and chiefly by the latter : the up|*r prong of the lupraicapula 

 again bifurcates to unite with both paroccipital and mastoid ; the long and slender lower prong 

 is joined by suture to a lateral process of the banioccipital. 



The fire anterior vertebne of the trunk are anchylosed together and to the basioccipital by 

 a continuous hypapophysial plate developed from their under part, which is perforated 

 lengthwise for the beginning of the aorta, and expands upwards and outwards where it is 

 closely applied to the under surface* of the co-expanded parapophyse* of the same vertebra-. 

 It is remarkable that the concave articular surfaces of the bodies of these coalesced vertebra 

 have been preserved by a retention of the interposed gelatinous remains of the primitive 

 ' notochord.' The body of the atlas is short ; it is quite detached from its broad parapophy- 

 se*, which are wedged between those of the occiput and axis : they are much expanded, and 

 terminate backwards each in a pointed process. The body of the axis is six times as long as 

 that of the atlas ; but the apirai of the two deep hollow terminal articular cones meet in the 

 centre : it* parapophysis is expanded, like that of the atlas, to which it articulates anteriorly 

 by a suture, as it does posteriorly to the co-expanded parapophysis of the third vertebra. The 

 gradual resumption by the para|>ophyses, in the succeeding vertebra-, of their normal form 

 and position establishes the serial homology of those of the three posterior cranial vertebne, 

 which parapophyses have received, on account of their special modifications, the special names 

 of ' paroccipital,' ' mastoid,' and ' postfrontal ' bone*. The axis, also, sends upwards a strong 

 spinous process : the bodies of the third and fourth vertebne are elongated like that of the 

 axis, but in a lew degree. That of the fifth vertebra is singularly modified ; its anterior half 

 presenting the long and slender character of the antecedent vertebne, whilst the posterior half 

 is suddenly shortened and developed in depth and breadth, like the short and broad centrums 

 of the succeeding free vertebne of the trunk. The nerves quit the spinal canal by directly 

 perforating the neurapophyses, and the anterior roots separately from the posterior ones, as 

 in the similarly coalesced vertebne forming the sacram of birds. 



Presented by //. N. Turner, jun., Esq. 



76. Tin- cranium, or neural arches, of the skull, with the three anterior trunk-ver- 



tebra? of a large Siluroid fish. 



The bodies of the vertebne of the trunk and occiput have been horizontally bisected, ex- 

 posing the biconical cavities between them. They are, however, firmly interlocked together 



I 



