﻿200 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



the igneous rocks and calcium and magnesium from the limestones and 

 dolomites. The result would be a very fertile soil containing an ample 

 supply of all the minerals necessary for a luxuriant and nutritious 

 vegetation. In view of the above considerations, it can be concluded 

 that this region offered a very nearly optinuim environment for 

 herbivorous animals, and that factors which would seriously disturb 

 the endocrine balance were absent. 



The estrogens and androgens (sex hormones) (Goldzieher, 1939, 

 p. 289) and the hormone of the adrenal cortex (ibid., p. 93) have an 

 inhibitory effect on the thyrotropic hormone of the anterior lobe of the 

 l)ituitary, which results in decreased activity of the thyroid. Although 

 estrogens and androgens are present in nearly all foods (ibid., p. 743) 

 and gTowing plants, and do not appear to be altered by the processes 

 of digestion, they are not secreted in quantity by the gonads till a 

 short period before puberty. It is believed that only a portion of 

 these hormones taken with food find their way into the blood stream. 

 It has been shown that these hormones are inactivated in the liver 

 and are rapidly destroyed by oxidation in the lungs (ibid., p. 748), 

 M'hich gi'eatly reduces their effectiveness when administered by mouth. 



In view of the role played by the thyroid in the formation of the 

 teeth by its effect on metabolism and growth, the low estrogen and 

 androgen content of the blood while the deciduous teeth were being 

 formed could result in the advanced type of teeth. That the third 

 permanent premolar is more advanced than the fourth in some species 

 of Hyracothenum (Granger, 1908) may be due to the sudden increase 

 in the estrogen and androgen content of the blood during the interval 

 between the formation of these two teeth. If the appearance of the 

 physiological brake on the thyroid furnished by the secretions of the 

 gonads and the adrenal cortex were postponed until after the deter- 

 mination of the form of the permanent teeth by the growth of the 

 tooth germ, it is conceivable that the advanced type of tooth would 

 result. In view of the antagonism between the gonads and the thyroid 

 (Goldzieher, 1939, p. 94), such a deferment could be the result of 

 mild hyperthyroidism. 



Family BRONTOTHERIIDAE 

 Genus LAMBDOTHERIUM Cope 



LAMBDOTHERIUM POPOAGICUM Cope 



U.S.N.M. No. 18464, right P-^NP (loc. No. 4SFR76) ; U.S.N.M. 

 18463, right P'-M^ (Shoshoni Reservoir) ; U.S.N.M. No. 18465, loose 

 upper teeth (loc. No. 48FR75). 



This genus is generally accepted as tlie index fossil for the Lost 

 Cabin faunal zone of the Wasatchian, Lower Eocene. Unfortunately, 

 tliis material is too fragmentary to add anything to our knowledge of 



