MEMBRANE BONES OF SKULL 581 



Just as the roof of the skull is coverc;d with dermal bones, so 

 its floor becomes protected by dermal bones originating in the 

 skin of the mouth. There has been much doubt about the homology 

 of these, but it now seems that there are in reptiles an anterior 

 pair, the vomers, underlying the region of the nasal capsules, 

 and posterior to these a single parasphenoid. The vomer of 

 mammals is a small bone formed by fusion of two lateral elements 

 in front of the presphenoid. The parasphenoid is absent from 

 mammals except for vestiges in a few species. 



It will be seen by comparing actual skulls with the diagrams 

 in Fig. 449 that there is a general tendency to reduction of the 

 dermal bones ; intertemporal, supratemporal and tabular, for 

 example, are absent from all modern reptiles. In the Anura the 

 reduction has gone very far, so that only the nasals over the 

 nasal capsule, and the frontals (usually called frontoparietals) 

 are present. In the urodeles the reduction has not gone quite 

 so far, as prefrontal and parietal are present also. By contrast, 

 the dermal bones of the palate, vomers and parasphenoid, are 

 large. 



THE VENTRAL SKULL 



Below the brain-box and sense capsules lie several skeletal 

 elements which, as we have seen above, have a fairly clear 

 segmental origin. Collectively they may be called the visceral or 

 ventral portion of the skull, but in the tetrapods their posterior 

 portions have no connection with the cranium and are situated 

 outside the head as generally understood, while some of the 

 anterior bones become secondarily and intimately associated 

 with the auditory capsule. 



Apart from the rather doubtful derivation of the trabecular 

 and polar cartilages from the segmental elements, the most 

 anterior of these are the supports of the jaws, which are derived 

 from the second or mandibular segment (Table \ II. i^. 371). 

 In the embryo there is a single upper-jaw cartilage on each side, 

 called the palatoquadrate, or palatopterygoquadrate, bar. and 

 articulating with each of these to form the lower jaw a single 

 Meckel's cartilage. A similar condition is found in the adult 

 dogfish. Behind this comes a series of skeletal supports, the 

 hyoid in segment 3 behind the spiracle, and then a branchial 

 or visceral arch behind each gill slit. The primitive division of 



