AKT. s. THE GENERA DESMOSTYLUS AND CORNWALLIUS HAY. 3 



would be the fourth premolar and the tooth immediately behind it 

 the second molar, while the molar shown at a distance behind would 

 be the third. 



In 1923 * the writer proposed the new name Gomwallius^ based on 

 DesmostyluB sookensh Cornwall.^ Through Director Francis Ker- 

 mode, of the Provincial Museum of Natural History, Victoria, Brit- 

 ish Columbia, the writer received for examination both of the teeth 

 figured by Cornwall. These teeth are illustrated on the plates (pi. 

 1, figs. 4, 5, and pi. 2. figs. 1-5). 



It has occurred to the writer that these teeth may be milk teeth 

 of Desmostylus and that the larger one (pi. 1, figs. 1, 2, pi, 2, figs. 

 4, 5) may correspond to that called in my paper of 1915 the first 

 molar. The tooth is of appropriate size. The length is 48.5 mm., 

 tlie width 34 mm. The corresponding dimensions of what the writer 

 regarded as the first molar ^ were estimated to be in length 40 mm., 

 and width 28 mm., but the measurements were taken at the somewhat 

 narrowed base of the tooth. The low crown of the tooth of C. soo- 

 kensis accords with the idea that it is a milk tooth. 



it seems certain that the tooth which in my paper of 1915 (pi. 57) 

 was designated by the numeral 21 is either a milk molar or the pre- 

 molar which would follow it; also that it corresponds in position to 

 the tooth Pm of Yoshiwara and Iwasaki's plate 2, and to the small 

 tooth of their plate 3, figure 4. The Oregon skull belonged evi- 

 dently to a younger animal than did the Japanese skull. In the 

 latter it seems that the premolar, if such it is, had not yet been 

 pushed out to the level of the molar behind it. In the Oregon speci- 

 men it is possible that the milk tooth 21 had not yet been replaced. 



As already said, the tooth designated 23 was lying immediately 

 above the greater part of the tooth 22. The former might, there- 

 fore, be taken as fourth premolar. Because of the great size of the 

 tooth and the number of its columns, this appears wholly unlikely. 

 The tooth 23 must be a molar. 



We may possibly get an explanation of the dentition of Desmo- 

 stylus from that of the mastodons. In some of these the premolars are 

 retarded in their development, in others even wholly suppressed. 

 The suppression of premolars began with the hindermost one. The 

 same modification of the dentition has been demonstrated in the 

 iSirenia. In the earliest members of the group, as Protosvren fra.ssi,'' 

 the tooth formula was the typical one, i. 3, c. i, pm. 4, m. 3. Accord- 

 ing to Andrews ^ the dental formula was the same in Eotherium 

 aegypticum. Abel tells us that since the middle Eocene there has 



* Pan-Amei-. Geologist, vol. 39, p. 106, text-fig. 4. 



^Canad. Field-Naturalist, vol. 36, p. 122, 4 figs. 



« Proc. U. S. Nat. Mus.,. vol. 49, pi. 57, 23. 



' Abel, Jahib. Min., Geol.. Pal., 1906, vol. 2, pp. 50. 51. 



^Cat. Tert. Vert. Fayum, 1906, p. 207. 



