348 SYNOPSIS OF THE VERTEBRATE FAUNA OF THE PUERCO SERIES. 



history of its own, and to confirm the Flowerian idea that the deciduous dentition is 

 an addition to mammalian development, and not a survival of reptilian conditions. 

 They also show that the proposition of the same author that the Mai'supialia have 

 four true molars is untenable, as I have previously claimed. 



Fragments of skeletons of the smaller Periptychidse are not rare in my collec- 

 tion. I have been unable so far to fix their species, and as they present no im- 

 portant diffeiences from the corresponding parts of Periptychus, I do not now 

 describe them. 



The mandibular dentitions of the species of the smaller Periptychidse are very 

 much alike, and they are with difficulty distinguished one from another. I give the 

 following table to facilitate their determination. 



I. Anterior external cusp of true molars extended forwards, but not incurved. 



a. Premolars elongate and compressed. 



Molar series about .033 m Haploconus xiphodon. 



aa. Premolars shorter, oval in longitudinal section. 



Molars .025 B. angustus. 



.032 E. lineatm. 



" .044 H. cornicidatus. 



II. Anterior external cusp of true molars witli anterior ridge directed inwards to a more or less developed fifth 



cusp. 



a. Premolars robust, with wide section. 



Premolars with oval section ; true molars .010 U. entoconns. 



Premolars with oval section ; true molars .009 H. cophaier. 



Premolars nearly round in section ; molars .019 HejiiithlcBus kowalevskianus. 



aa. Premolars narrower, with strong anterior cusp. 



True molars .018 Am'sonchus scctorivs. 



True molars .014 netnithlavs apiculatus. 



True molars .012 Anisonchus gtllianiis. 



True molars .011 A. agapetillus. 



aaa. Premolars much compressed and elongate. 

 True molars .040 A. mandibtilaris. 



The mandibular dentition of Ayiisonclms coniferus is not sufficiently Avell known 

 to be introduced into the table. 



HAPLOCONUS Cope. 

 Amer. Naturalist, 1882, p. 417 ; Tertiary Vertebrata, 1885, p. 41.5. 



The penultimate and last superior milk molars of this genus have been described 

 above, together with the inferior milk molars in the genus Anisonchus. It results 

 from these observations that the i)eculiar form of last and penultimate infei-ior pre- 

 molars which I have observed in the Haploconus xiphodon are permanent teeth, as I 



