914 



HISTORICAL GEOLOGY. 



American, is the increasing size of the brain with the progress of the Tertiary. 

 In most of the earliest Eocene species the brain was extremely small, and 

 especially the cerebral or anterior part; that of the Dinoceras ujight have 

 been drawn entire through the cavity of the spinal cord. This point is 

 illustrated in figures 1541-1543 (from Marsh), representing the skulls, 

 reduced to a common length, with the brain cavity : of the Eocene Dinoceras 



1541 



1541-1543. 



1542 



1543 



Illustrations of the sizes of brains in successive genera of Ungulates. Fig. 1541, Dinoceras (Eocene) ; 1542, 

 Brontotherium (Miocene) ; 1543, Modern Horse. From Marsh. 



(Fig. 1541), the Miocene Bi-ontotlierium (Fig. 1542), and the modern Horse 

 (Fig. 1543). The Horse has a brain more than eight times the bulk of that 

 of the Dinoceras. It is seen in these figures that the posterior part of the 

 brain, as Marsh observes, has undergone little change of size, the enlargement 

 having been eminently in the cerebral portion. The principle has necessarily 

 its exceptions, since size is not the element of most importance in a brain. 

 Marsh has further shown that the principle is exemplified in fossil Birds, and 

 also in the Dinosaurian group of Reptiles. 



In addition to relics of Eodents in tlie form of bones and teeth, there are, 

 in the Niobrara region, what have been supposed to be burrows of some 

 species of Rodent. They were described as probably fossil Sponges by 

 E. H. Barbour ( Univ. Studies, Nebraska, 1892, where many excellent figures 

 are given, some showing specimens in place). They stand vertically, in 

 large numbers, in the Miocene of the region, some of them 8 or 9 feet in 

 height. Each usually ends below in a long horizontal or oblique chamber. 

 Dcemonelix is a Greek form (abbreviated) of the popular name "Devil's 

 corkscrew." The figure includes two views of one of the specimens, the 

 vertical spiral of which is 53 inches high, and the oblique basal portion 

 76 inches. 



