230 PROCEEDINGS OF THE ACADEMY OF [1894. 



with its fellow a single sharp, almost spear-shaped, point for piercing; 

 symphysis of mandible long, straight or nearly straight, and usually 

 bent up at a sharp angle with ramus; the two posterior enamel folds 

 of m 1 and •> simple, reaching completely across the tooth from side 

 to side as in m .{ ; each re-entrant angle corresponding to a salient 

 loop on opposite side; investing enamel walls parallel, the included 

 dentine forming a continuous narrow band of equal breadth through- 

 out. 



Ptyssophoriis and Hodomys agree in the following characters : — 



Crown of m ,{ shaped like the letter S placed lengthwise of jaw; 

 projecting part of lower incisor nearly straight, slender, and forming 

 with its fellow a single sharp almost spear-shaped point for piercing; 

 symphysis of mandible long, straight or nearly straight, and usually 

 bent up at sharp angle with ramus. 



Ptyssophoriis and Ilodomi/s differ in the following characters: — 



Genus PTYSSOPHORUS Ameghino. 

 (Fig. Ic.) 



The two posterior enamel folds 

 of m 1 and m ^ simple, reach- 

 ing com})letely across the tooth 

 from side to side as in m :; ; each 



Genus HODOMYS nob. 

 (Fig. ]c.) 



All enamel folds of m i and 

 m 2 reaching only about half- 

 way across tooth ; each re-entrant 

 angle corresi)onding (at least in 



re-entrant angle corresponding to j young) to re-entrant angle of 

 a salient loop on opposite side ; opposite side ; investing enamel 

 investing enamel walls parallel, j walls alternately divaricating and 



the included dentine forming a 

 continuous narrow band of equal 

 breadth throughout. 



approximating, the included den- 

 tine broken into disconnected 

 parts. 



The principal differences between the lower molars of PtyssopJwriis 

 and those of Hodomys are, that in Ptyssophoriis most of the enamel 

 folds reach all the way across the tooth ; the enclosed dentine is of 

 nearly equal width throughout ; the anterior loop of the first molar 

 has an additional lobe, and the first and second external loops of the 

 middle molar are more crowded and less distinctly separated (much 

 as in Sifpnodon^). The only one of these differences of more than 



"^ The (Miaiiiol iwttcrii of tlic crowns of in -j and :! ol' /'/vss(>/>/ii>rt/s /■/f<^>{i)ts 

 (fig. Ir) is almost identical willi that of yoiuifj siiccinicns of sonic liviiis species 

 of Sifftuodnii (fifT. 1"'). l>nt the cliaracter oi' the teeth isdiflereiit : In S/o/i/odo/i 

 the crowns have liardly left the lulierciilate condition ; tlu' enamel is of uiie(|ual 

 thickness, tlie Ioojjs are closely aijpresscd, and the re-entrant anjjlcs are of super- 

 ficial depth vertically ; in J'/yssoplKiriis the crowns are truly prismatic, perfectly 

 flat on top, the loops widl spaced, and the re-entrant angles reach from crown to 

 alveolus. 



