Anatomical Structure of certain of the Cetacea. 411 



analogous to what we find in the porpoise and dolphin, and the 

 squamous or ascending plate of the occipital bone is imperfectly 

 ossified, and very thin. The jaw where the developed tooth is 

 contained is slightly reticulated on the upper surface. The 

 symphysis of the lower jaw has been originally united by syn- 

 condroses, and a shallow groove runs along, for a short distance, 

 the upper margin of the lower jaw, obviously analogous to the 

 deep groove in the porpoise, dolphin, and many other Cetacea, 

 containing in them the sharp conical teeth, but, as is well known, 

 there are no teeth of this kind in the narwal. The maxillary 

 bones are loose and spongy; the inter-maxillary firmer and 

 denser, more resembling in structure the inferior maxillary 

 bones. 



Mr HUNTER'S admirable account of the dentition of the 

 ordinary Cetacea may be corrected apparently in one point; 

 the groove, or elongated cavity for the reception of the young 

 teeth, cannot be formed by the sinking down of the teeth in it, 

 for the teeth are already deeply imbedded in it in the foetus. 

 In many species of animals, moreover, the osseous partitions of 

 the alveolar cavities are by no means complete. It is probable, 

 therefore, that the germs of the teeth are developed in this 

 elongated alveolar cavity, much in the same way as they are in 

 other Mammalia ; but, as Mr HUNTEII remarks, they do not 

 succeed each other by germs placed above or below each other 

 in the jaw (according to the jaw spoken of), but rather from be- 

 hind forwards, the anterior ones, together with the portion of 

 the maxillary bone carrying them, gradually wasting away by a 

 law in the economy of the animal. The inter-maxillary bones 



carry -r- teeth apparently in many of the Cetacea, as in all the 



porpoises. 



In the adult animal, and no doubt in many others of the Ce- 

 tacea, this elongated groove for the reception of the teeth may, 



VOL. XI. NO. II. 3 F 



