Dive-oj>meul of tite Teeth of ihe k^oricidie. G21 



liavc been fotiiul, but tlic fragments of the upper jaws arc 

 rather spare. 



However, tlie most important result of this investigation 

 is that the vast yap irhich has hitherto existed, ivith retjurd 

 to the uiiinber of incisors, between the poh/protodont Muininalia 

 — the Marsiipialia included — aiid the Monudelphia is noin 

 bridi/ed over : for tnj this inrestii/alion the existence of inter- 

 mediate forms has been shon'n, f/ernis of more than three 

 incisors having been found in ntonodelphic heterodont mammals : 

 !Soricid(.e. 



Among now living Mammalia the iNIarsupialia alone are 

 characterized by polyprotodonty and hcterodonty. But in 

 luy opinion these points of agreement do not warrant 

 the conclusion that the SoricicUe are descended from now 

 living jMarsnpialia*. 



The said points of agreement — and possibly others too — 

 seem to nie rather to argue in favour of the view that the 

 JSoricidre and the Marsupialia have a common origin. 



With respect to the tooth-rudiment ^Ix found in Croci- 

 dura russula 1 have already pointed out that it can hardly 

 he regarded as a premolar, because such a view would 

 involve the supposition that a progenitor once existed 

 having a greater number of premolars than the known recent 

 and Mesozoic mammals have. 



It is true that in two genera, Amphitherivm and Peramus, 

 the number of premolars exceeds lour, but they are regarded 

 by Osborn f as exceptions : " The almost invariable presence 

 of four premolars among the Mesozoic and recent mammals 

 is a very difficult fact to explain. This genus \_Peranms'] and 

 apparently Amphitherium are among the few exceptious,^^ 



In su])port of an opposite opinion, i. e. that the tooth- 

 germ Mx is a molar, the following facts may be alleged : — 



1. A tooth-rudiment evidently belonging to the milk- 



dentition has been found, but no trace of a successor is 

 distinguishable. All the permanent teeth are, however, 

 already differentiated. 



2. In the ]\[esozoic mammals the number of molars surely 



exceeded three. If really the ancestors of the Soricidic 



* Ziehen, " Das Centralnervensystem der Moiiotrenieii uml ^laisu])!- 

 alier," p. 175: Deiikscbiil'teii der Med.-nalurw. Gesellscluil't zii .leiia, 

 Jkl. iii. T. 1 (1897). 



t Oriborn, H. F., "Additional Observations upon the Structure and 

 Clas^itication of the Mesozoic Mammals," Proc. Acad. Nat, Sci. Philu- 

 delphia, 1888, p. 296. 



