642 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Eostral part of skull very sleader and attenuate, with nearly straight 

 and horizontal upper profile far out of line with the profile of the swollen 

 cerebral part. Interorbital constriction at the maximum. Bridge over 

 the auteorbital foramen comparatively narrow. Coronoid process of jaw 

 slender, tilted forward ; angular process comparatively short, extremely 

 slender, and nearly horizontal. Tail scarcely or not shorter or much 

 longer than the head and body, without a decided keel of hairs along 

 the under side. Ear distinct, the auricle directed backward (as usual). 

 Feet large, fringed witb long, straight, stiffish hairs in regular series; 

 the hinder feet especially well developed and natatorial, about one-fourth 

 as long as the head and body. Pelage long, soft, and thick, to resist 

 water. Of large size and highly aquatic habits. 



Type. — JSleosorex navigator Cooper. Peculiar to North America, where 

 it replaces Crossopus of the Old World. Includes the Water-shrews of the 

 Western Hemisphere. Although the strict rejjresentative of Crossopus, 

 it differs decidedly from thelatter in cranial, dental, and external charac- 

 ters. In Crossopus there are but 30 teeth, the unicuspids being only 4, 

 the three anterior ones regularly graduated in size, the fourth minute ; 

 the upper incisors appear to lack tlie lobe on the inner edge, the posterior 

 hook is smaller, and the under incisors have but one denticulation (in 

 the only specimen before me). The skull is much broader, as shown 

 especially between the orbits and at the interpterygoid space. The con- 

 tour of the palate is more broadly pyriform ; the rostral part of the 

 skull is much less attenuate, with wider bridge over the anteorbital 

 foramen. The mandible as a whole is stronger, with stouter, more 

 nearly perpendicular, coronoid process, and more obliqu© angular pro- 

 cess. The tail has a prominent keel of stiffish hairs along the under 

 side, not developed in Neosorex. I am unable to compare the ears. The 

 feet are very similar, though the fimbriatiou is less conspicuous. In 

 general appearance, Neosorex navigator and JV. pahistris are quite like 

 Crossopus fodiens, all these Water-shrev/s being large, long-tailed, large- 

 footed species, sharply bicolor, blackish above and whitish beneath. 



plied in the use of the term " premolars ", applied by Baird to these teeth. The Shrew's 

 teeth are conveniently separable, for purposes of ordinary description, into three sets, 

 namely : — (1) The enlarged and peculiarly modified single incisor, on each side, above 

 and below ; (2) A series of varying number of small unicuspid teeth, intervening be- 

 tween the incisors and the molariform teeth ; (3) The molariform teeth, always four 

 above on each side and three below on each side. The anterior one of these last on each 

 side above is homologically a premolar, though it is like the true premolars in appear- 

 ance ; the homologies of some of the uuicnspid teeth are in question. All the American 

 Sorieidce (like all the Shrews as far as known) have the same number of under teeth, 

 namely, 12. The differences in the number of the teeth arise from the presence of 3, 4, 

 or 5 "unicuspids". The following formula covers the numerical composition of the 

 teeth of all American shrews^ without being committed to their homologies : — 



16 or 18 or 20 



12 



= 28 or 30 or 32. 



