262 PROCEEDINGS OF THE ACADEMY OP 



partially completed, viz., that of the camels. I have already indi- 

 cated the antecedent relation in which the miocene genus Poebro- 

 therium stands to the existing camels in the structure of the limbs 

 and teeth, as well as the intermediate position occupied by Pro- 

 camelus in the existence of the incisor teeth. It now remains to 

 point out the relations determined by the structure of the feet in 

 Procamelus and the dentition in Pliauchenia, a new genus dis- 

 covered in the Loup Fork beds of New Mexico. Commencing with 

 the earliest genus, Poebrotherium, we have the molar teeth 4 3, 

 as in the primitive mammalia generally. There are but two elon- 

 gated metacarpals, which are not united into a cannon bone, the 

 lateral ones being rudimental, while the carpals are of the number 

 characteristic of the mammalia of all the orders with numerous 

 toes, namely, seven. In Procamelus of the succeeding formation, 

 the molar formula continues to be 4 3, but the posterior teeth 

 are more prismatic in form than in the miocene genus. The incisor 

 teeth are present, thus displaying the primitive character of the 

 class generally ; though as these teetli are early shed, an approxi- 

 mation to the edentulous condition of existing ruminants in this 

 part of the mouth is apparent. In the feet, the approximation 

 to the existing Camelidse is greater than in the dentition. Thus 

 the lateral rudimental metacarpals of Poebrotherium have disap- 

 peared, and with them the trapezoides of the carpus. The magnum 

 remains distinct, while the middle metacarpals are united at full 

 age into a cannon bone. In the contemporary genus Pliauchenia, 

 a further modification of dentition is observed. As above stated, 

 the molars of Procamelus number f ; in Camelus they number 

 3 f ; in Pliauchenia we have the intermediate condition ? | . 

 The end of the series is seen in Anchenia, where the formula is f . 

 It has been observed as a remarkable fact, that North America 

 should present us with the most complete history of the succes- 

 sion of genera which resulted in the horse, and yet should have 

 received this animal by importation from Europe. Nevertheless 

 the more prominent genera of this series have been obtained in the 

 European formations; especially Anchitherium and Hippotherium. 

 But as regards the Camelidse, the genera above described are ex- 

 clusively North American ; no well-determined form of this group 

 having been found in any formation of the Paloearctic region up 

 to the present time. Until such are discovered, there will be much 

 ground for supposing that the camels of the old world were de- 

 rived from American ancestors ; while the presence of the llamas 

 in the existing South American fauna, indicates the absence there 

 of the conditions which caused their extermination from North 

 America. 



Living Organisms in the Pulp Cavity of Teeth Mr. C. N. 



Peirce remarked that, in making some investigations on the 

 organisms found within the oral cavity, he noticed what might be 



