MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 83 



of the dorsal surface of the arch. The eleventh, twelfth and thirteenth 

 spines are similar, but although only slightly elevated, they are definitely 

 directed forward instead of backward. The thirteenth is slightly lower 

 than the eleventh and twelfth and closely resembles the spines of the 

 succeeding lumbars. 



The position of the so-called center of motion sharply distinguishes 

 Ccenokstes from all living American polyprotodonts and aligns it with 

 the majority of Australian forms, as well as with most higher mammals. 

 In Didelphis, Marmosa, Philander, and Peramys there is practically no 

 anticline since the spines of all the thoracic vertebrae and of all the 

 lumbars, except the last or at most the last two, are inclined backward. 

 The only other marsupials having this characteristic are the koala, the 

 wombats, and certain of the phalangers. In all the Australian poly- 

 protodonts and at least in the macropods of the diprotodonts the center 

 of motion is in the posterior thoracic vertebrae and in most of these 

 forms the anticline is very marked. 1 This is also the case in C&nolestes 

 and in the Patagonian fossil form Cladosictis. Whether the condition in 

 the didelphids is palaeotelic or not is problematical. Essentially the 

 same condition is found in the monotremes and edentates, and it is 

 usual among reptiles, whereas among higher mammals it is rare and 

 generally correlated, perhaps secondarily, with high specialization, as 

 in seals and anthropoids. In practically all insectivores, rodents, bats, 

 carnivores, ungulates, and in Primates, except the higher apes and man, 

 there is a marked convergence of vertebral spines to a center of motion. 

 In agreeing with the majority of mammals in this respect, therefore, 

 C&nolestes at least shows a lack of affinity with the didelphids. 



The first nine thoracic vertebrae are essentially similar in form. 

 The arches and pedicels, although progressively increasing in length, 

 are relatively narrow and each vertebra is separated from the succeeding 

 one dorso-medially by a considerable space between the projecting 

 zygapophyses. Each has a transverse process bearing a facet for the 

 tubercle of a rib. The tenth begins to assume the character of the 

 lumbars. It has only a trace of a facet for the tubercle of a rib, its spinous 

 process is low and ridgelike instead of spinous, its laminae and pedicel 

 are broad and its body is less compressed. The eleventh has these 

 characters further developed. There is no trace of a lateral rib attach- 

 ment and posteriorly there is a tiny pointed accessory process below the 

 postzygapophysis. In the twelfth and thirteenth this accessory process 

 is slightly larger and in these the prezygapophyses begin to turn upward, 

 but there is only a slight rudimentary mammillary process. 



1 In the single skeleton of Sarcophtlus examined there is a double anticline, one 

 at the eleventh thoracic and another at the third lumbar. 



