THE SKELETON OF THE VERTEBRATE HEAD. Ill 



from the ordinary form in which the centrum passes across, 

 which, therefore, I also occasionally find it convenient to in- 

 dicate as the diacentric form of sclerotome. The passage 

 from the diacentric to the catacentric form is exemplified in 

 the ethmoidal sclerotome, the haemal arch of which, consisting 

 of the pair of maxillary bones, enters into the formation of 

 the nasal passages. The centrum of this sclerotome has as- 

 sumed the form of a more or less compressed plate, which, 

 while it retains its lateral connection with the neurapophyses, 

 extends at the same time more or less upwards into the 

 neural space, and downwards between the nostrils, which, 

 under this sclerotome and the one behind, consist of a mesially 

 bisected haemal cavity. 



The anterior terminal sclerotome in the non-proboscidian 

 mammals is cartilaginous and catacentric. Its neuro-haemal 

 chambers are closed in front by the junction of the anterior 

 margins of its neural and haemal elements. In consequence, 

 too, of the position of the external nostrils, which, as metaso- 

 matomic openings, are situated between the haemal elements 

 of this sclerotome and those of the sclerotome immediately 

 behind, its haemal elements are tilted forwards, so that towards 

 their junction with the neural elements their sternal margins 

 are continuous with the dorsal line of the nose. In the more 

 developed forms of this sclerotome, from one to three haemae- 

 tinapophyses on each side enter largely into its arrangements. 

 In the proboscidian mammals, instead of being greatly 

 developed, as might t naturally be expected, this sclerotome is, 

 on the contrary, much simplified. In the tapir the haema- 

 pophyses have disappeared, while in the elephant the neura- 

 pophyses alone exist in a comparatively undeveloped form. 

 I believe, however, that it will ultimately be admitted, that 

 the proboscis is not a mere elongation or development of the 

 external nose, like the pseudo-proboscis of the bear, racoon, 

 and coati, but a syssomatome. 



