276 ON THE STRUCTURE OF THE SKULL. 



that of tlie jaws, is soon arrested, and in addition the position 

 and relations of some of the cranial bones become altered. In 

 the smallest foetuses of the lesser Fin-back (Pterobalmna minor), 

 for example, the parietal region is occupied by the inter- 

 parietal bone and the great fontanelle which lies in front of it. 

 In larger foetuses the fontanelle becomes closed by the progressive 

 backward growth of the frontals, but the extension of the bones 

 does not cease with their contact. The parietals grow over the 

 inter-parietal and spread over it until they meet in the middle 

 line. Hence the interparietal is eventually visible only in the 

 interior of the skull. Anteriorly, the parietals grow over the 

 frontals almost to the same level as the nasals, and thus conceal 

 the share which the frontals take in the formation of the roof of 

 the skull. But, at the same time, the supra-occipital extends 

 from behind over the parietals ; so £hat, at length, in that region 

 which, in the youngest foetus, was covered only by the inter- 

 parietal, three bones — the inter-parietal, parietal, and supra- 

 occipital — are superimposed. 



The skulls of the other great division of the Cetacea, the 

 Delphinoidea — or Dolphins, Porpoises, and Cachalots — are almost 

 all distinguished by their very marked asymmetry. 



In the Cachalot, or spermaceti Whale (Physeter), for example, 

 the right premaxilla is much longer than the left, extending far 

 back upon the right frontal, while the left does not reach the 

 left frontal ; the left nostril, on the other hand, is much more 

 spacious than the right (Fig. 110, A). On the base of the skull 

 (Fig. 110, B) the pterygoid bones unite in the middle line and 

 prolong the palate, as in Myrmecopliaga and Omitliorliynchus. 

 AVhen they and the palatine bones are removed, the axis of the 

 lower part of the ethmoid is seen to continue that of the basi- 

 cranial bones, which are, as usual, quite symmetrical. Supe- 

 riorly, however, the ethmoidal plate is twisted over to the left 

 side, and deeply grooved on the right side to form the inner 

 wall of the small right nostril. 



The vomer, which embraces the ethmoid and the presphenoid 

 below, is also asymmetrical posteriorly, presenting a long and 

 shallow lateral excavation, on the left side, and a short and deep 

 one on the right. The maxilla? are correspondingly unsym- 



