OF DIDELPIIYS VIRGINIANA. 47 



resulting in a flattened, cutting surface that looks forward and inward. The long axis 

 of the tooth is almost vertical : the root is unicuspid and rather longer than the exposed 

 portion. An average incisor, root and all, is a fourth of an inch long. 



A considerable interval occurs between the incisors and the canine, for the reception of 

 the lower canine. The upper canine is of remarkable size ; tliis formidable weapon is as 

 large, comparatively, as — even if it does not exceed — the tusk of the Felidce or Canidce. 

 The whole tooth is over an inch in length, in an adult male ; rather more than half its 

 length protrudes from the socket. In the female, the tusk is both relatively and abso- 

 lutely smaller. The large root causes a protuberance of the jaw-bone along its course ; the 

 base is abruptly truncate, with so large a cavity that the rim of the opening is a thin, 

 sharp edge. The canine is compressed laterally and curves regularly from base to tip in 

 the arc of a large circle ; so, although the course of the root in the alveolus is obliquely 

 forward, the point of the tooth is directed downward, and a little backward. In an un- 

 worn condition, the tooth tapers to an extremely sharp point, which reaches below the 

 edge of iliG mandibular alveolus. The opossum's bite is severe, and inflicted mainly by 

 the four canines, which the animal can easily thrust through stout boot-leather. 



The first premolar is much smaller than the other two, from Avhich it is separated by a 

 wide interval. It is inserted immediately behind, in juxtaposition with, the canine, and 

 almost appears like a basal snag of the latter. It has two very long, slender, parallel 

 roots, curving slightly backward ; these roots do not join to form the body of the tooth 

 until fairly outside the socket ; a slight groove is continued downward from their point of 

 union. The exposed portion of the tooth is very small — scarcely half as long as the 

 root ; it tapers rapidly to a single sharp point, with no noticeable basal dilation. The 

 second and third premolars successively enlarge ; the exposed portion correspondingly 

 preponderates over the rooted part ; the latter is deeply cleft into two prongs as before, 

 but these are stouter, straighter and divaricating a little from each other. These teeth 

 are unicuspid, like the first premolar, but heavier, blunter, with a thicker base ; and at 

 the base of the posterior border a well developed snag is given off. The second premo- 

 lar touches the third by means of this basal snag : the third is opposed to the first molar 

 by the same means. 



The first three molars arc very similar to each other in essential characters, and in- 

 crease in size from first to third ; the fourth, or wisdom tooth, is smaller than either of 

 the others, and diflerently shaped. It is cut later than the rest; I find it scarcely pro- 

 truding from its socket in an animal two-thirds grown. The cutting of all the molars 

 appears from before backward. The molars are in mutual contact by their external 

 borders ; the triangular shape of the crown produces a large interval between contiguous 

 teeth along the inner border. The crown of the first three molars has the shape of a 

 right-angled triangle, with nearly equal base and perpendicular. The anterior border 

 proceeds straight inward toward the median line of the mouth ; thence with a rounded 

 termination obliquely backward and outward to meet the external border. Each of the 

 three corners of the teeth developes a moderate cusp : the middle of each side rises into 

 a smaller tubercle (that of tlie o])lique side being the largest) ; the grinding surface 

 surrounded by these elevations is irregularly concave. On viewing the molars from the 

 outside, each appears almost divided into two by a deep notch in the lower border, which 

 is opposite a similar notch in the upper border, formed by the late confluence of the fangs. 



