368 Annals of the Carnegie Museum. 



apparently to be completely equipped for an open country habitat? 

 Although a number of causes combined may be suggested, we may 

 never know the true one. In his admirable work, "The Age of 

 Mammals," Professor Osborn has called attention to observations by 

 Darwin, Gregory, Chestnut, and Willcox of the devastating effects 

 on animal life which have been brought about by great droughts at 

 certain seasons in different parts of the world. A long succession of 

 dry seasons in a given locality is certain to have a telling effect on the 

 fauna. An invasion of a new and destructive type of carnivore may 

 also be a potent factor in the extinction of certain herbivorous types. 

 We have not as yet clear evidence of the immediate ancestors of the 

 llama of South America. Although the precursors of the latter un- 

 doubtedly occupied the same general region in which Stenomylus 

 lived, we are not, as yet, ready to advance any explanation, or even 

 give a working theory as to why Stenomylus did not survive on the 

 vast plains of North America or migrate with other branches of the 

 CamelidcB. 



In an extensive collection of these fossil remains it is found, as in 

 the study of recent zoology, that the contour of the skull varies 

 greatly in the young and the fully adult. The direct side view of the 

 cranium of the young Stenomylus presents a characteristically juvenile 

 appearance (see Fig. i). The occiput is less angular than in the adult, 

 the brain-case as a whole is more ovate, there is a complete lack of the 

 sagittal crest, and a more rapid convexity fore-and-aft from the occiput 

 to the region between the orbits. The latter are very nearly as large 

 as in the adult specimens, while the nasal depressions are distinctly 

 less deep. The facial region is apparently shorter, so that the orbit 

 has a more anterior position in the immature skull. The horizontal 

 ramus of the lower jaw tapers more rapidly anteriorly and the vertical 

 portion is proportionally greater than in the adult. The dentition 

 is represented by the milk molars except p 1 ; the latter is seen in the 

 side of the maxillary upon excavating the bone. The canine is in 

 place. The premaxillaries are lost and the extreme anterior portion 

 of the mandible is also broken off. 



The skeleton represents an animal about half-grown (see Plates 

 XXI and XXII). The epiphyses of the centra of the vertebral column 

 were found dislocated and the ends of the limb bones were detached 

 by suture, though in their relative positions. The pelvis was also 

 found in sections, that is, with the ilium, ischium and pubis entirely 



