6 PEOCEEDINGS OF THE NATIONAL MUSEUM Vol. G8 



diameter at right angles with the width being at least 16 mm. (pi. 6, 

 fig. 2) ; in the capybara, only 8.9 mm. In the latter its base is in a 

 prominence lialfAvay down on the inside of the jaw and opposite 

 the rear of the first tooth; in the fossil it was as far backward but 

 higher up, apparently below the alveolar border about one-third 

 the width of the jaw. The length and width of the incisor can not 

 be determined. If the proportions were as in the capybara the 

 width was 20 mm. In front of the base of the premolar and well in 

 front of the masseteric ridge is a mental foramen whose diameter 

 appears to have been about 4 mm. In the capybara the foramen is 

 only 2 mm. in diameter, and it pierces the bone somewhat behind the 

 front end of the ridge. 



The writer is indebted to Gerrit S. Miller, jr., for some important 

 observations on this fossil. He suggests that in two respects the 

 animal is in a more advanced stage than is the capybara. One pro- 

 gressive feature is found in the elevation of the rear of the incisor; 

 the other is indicated by the reduction of the coronoid prooess. A 

 less progressive feature is found in the failure of the masseter muscle 

 to extend as far forward as it does in the capybara. 



In consideration of the differences between the Sinton rodent and 

 the existing capybara {Hydrochoerus hydrochoerus) it seems that 

 the former belonged to a line of descent which had parted from that 

 of the capybara, and that hence a new generic name is appropriate. 

 Neochoerus is chosen, as suggesting the swimming habits of the 

 animal and its relationship to Hydrochoerus^ the first part of the 

 name being derived from the Greek uecr, to swim, the remainder from 

 xotpo5, a hog. 



Accompanying the mandibular ramus above described is an upper 

 left third molar in a piece of the maxilla and a little of the pala- 

 tine. The length on the grinding face is 60 mm. ; the greatest width 

 24 mm. It is composed of 17 plates, as represented by the edges 

 exposed on the outer face. The best-preserved plates are 36 mm. high. 

 It appears that the palate of the Aransas River specimen was not 

 so flat as in the capybara. In this the roof of the palate, at the 

 middle of the hinder molar, makes an angle of 123° with the grind- 

 ing surface of the molar; in the fossil an angle of 138°. At this 

 inclination the roof rises a distance of 21 mm. to the fracture near 

 the midline. A 



In 1923 the writer described a species of Hydrochoerus founded 

 on a tooth discovered in the Pleistocene deposits at Charleston, South 

 Carolina, and this he called H. pinckneyi.^ Later in the same year 

 that tooth was further described and other figures of it were shown.® 



^Publ. 322, Carnegie Institution, Washington, p. 365, flg. 18. 

 6 Pan. Amer. Geologist, vol. 39, p. 103, pi. 7, figs. 3-5. 



