in the Foetal Mammalian Jaw. 421 



immediate ancestors of the mammals ; but there is reason to 

 believe that aiioniodonts and inaninials are diver']rin;; brandies 

 from a common origin, and which primitive group was 

 derived from the higher amphibians. The larva of the frog 

 and of most other am])liibian.s is provided, as is well known, 

 with a horny beak on the front of each jaw, wjiicli fulfils all 

 the requirements of teeth. In the siren the horny sheath is 

 retained throughout life. It is impossible to say with what 

 armature the jaws of the young anoinodonts were provided ; 

 but it is highly ])robable that they possessed beaks, as in 

 most of the groups of the order we find teeth playing a 

 secondary part. In Oudenodon teeth were entirely absent, and 

 the jaws have evidently been provided with a strong horny 

 casing. A similar condition existed in Dicynodon^ with the 

 addition of a pair of large maxillary teeth. In Endothiodon 

 we find the unique arrangement of a horny beak on the pre- 

 raaxilla and maxilla, with a row of small teeth in addition on 

 the maxilla internal to the horny edge. The theriodonts had 

 a row of teeth along the alveolar margin of the premaxilla 

 and maxilla, though it is probable that, as Owen believed 

 (Todd's Cyclop. Anat. and Phys., art. Teeth), these animals 

 were monophyodont, and in this differed entirely from the 

 ordinary reptilian type. In the remaining group of anomo- 

 donts, of which Fariasaurus is the type, we find a single 

 feebly develoj)ed set of teeth. 



As we find evidences of a horny beak in many of the anomo- 

 donts, or a feeble development of teeth, rendering probable 

 the existence of a horny armature in the young of this group, 

 which is most nearly allied to the mammals of which we 

 have remains, there is every reason to suppose that the pro- 

 liferated epithelium which arms the front of the fcetal 

 mammalian jaw is the exact homologue of the horny beak of 

 theanomodont and tlie remains of a functional beak possessed 

 by the young of the mammalian ancestor. These young were 

 probably furnished with a well-developed horny beak on 

 escaping from the ^gg, and, being probably aquatic in habit, 

 derived nourishment from sucking plants and decomposing 

 animal matter. 



The marsupials are probably derived from a subdivision 

 of this premammalian group, in which the horny beak of 

 the young was developed to a greater extent and retained to a 

 later period in life, aborting the secondary tooth-germs, and 

 resulting in the descendants being practically monophyodont. 



Taralga, New South Wales, 

 Jan, 20, 189r,. 



