NO. 1101. PllOCEEDINGS OF THE NATIONAL MUSEUM. 61 



incisor on botb sides. A skull from Fort Klamath, Oregon, has no first 

 upper lireraolar on eitlier side.^ 



I have not detected any reduction in the number of mandibular teeth. 



AGE VARIATIONS IN SOAPANUS. 



The variations dei)ending- on age which are of the most interest are 

 those aftecting the shape of the skull and teeth, 



.Youngish skulls, or those in which the teeth are unworn, have a high, 

 rounded brain case and smooth surface. The sutures, from the frontals 

 backward, are open, but these api)arently are never obliterated, even in 

 extreme old age. The facial sutures become closed at a comparatively 

 early age, and are rarely to be distinguished in skulls with worn teeth. 

 The sutures at the base of the cranium, on the contrary, close very 

 early, even before the milk teeth are all shed. 



As age progresses, while the inferior half of the brain case remains 

 unchanged, the supeiior half becomes depressed, especially in front, 

 where the parietals meet the frontals, so that the frontal sinuses have 

 the appearance of being elevated. 



The arcuate transverse occipital ridge, which appears to mark the 

 union of the occipital with an interparietal, becomes more pronounced 

 and moves forward, so that the "interparietal" is much narrower than 

 young skulls. This change is very striking and might easily be mis- 

 taken for a specifi(5 character. The ellii)tical capsule which covers the 

 snjierior vermis of the cerebellum becomes more prominent. 



All the ridges of the skull are, of course, more strongly developed as 

 age progresses. There is a distinct but low sagittal crest. The pos- 

 tero-external margin of the squamosal is thickened and the surface of 

 the maxillai immediately in front of the frontals is depressed. 



The changes in the form of the teeth are very striking. The unicus- 

 pidate teeth are at first long, slender, and sharp, and (in northern 

 specimens at least) separated by considerable intervals. As they 

 become worn there is an apparent increase in size, so that the intervals 

 between them are nearly or quite closed, and they are short and 

 massive. The small internal and anterior basal cusps of the last 

 upper premolar are usually obliterated, as well as the posterior cusp of 

 the first and second premolars. 



The changes in the molars are not less striking, ISTot only are the 

 cusps entirely worn off in age so that the inferior or grinding surface 

 of the teeth is nearly or quite Hat, but the shape of the cusps them- 

 selves is altered. The small accessory cusps of the upper molars are 

 obliterated, and the internal basal cusp is worn away not only at the 

 extremity but at the sides, so that it takes a new form and is much 

 narrower in proportion to the whole tooth than in unworn teeth. 



The lower molars suffer similar changes. In the last molar the 



'This is No. 1286, Merr. Coll., the type of my Sr.apanus dilatus. TLis species, based 

 chiefly ou tlie absence of the premolar, I now regard as invalid. (See p. 6-1.) 



