592 MORPHOLOGICAL METHOD AND EECENT PROGRESS IN ZOOLOGY. 



it mainly invohcs the mouth (as in all tiyhcs and batrachians), the hard 

 and sharp line between the Batrachia and Amniota may be expressed 

 by the formula that the former are arcluecraniate and stomatophysous, 

 the latter s^ncraniate and somatoph3'sous. 



There are allied topics which might l)e considered did our time per- 

 mit, l)ut one certain outcome of this is that there is an end to tlie notion 

 of a batrachian ancestr}^ for the Mammalia. And when, on this basis, 

 we sum up the characters demanded of the stock from which the Mam- 

 malia have ])een derived, Ave find them to ])0 precisely those occurring 

 outside the Manmialia in the Anomodont reptiles alone. Beyond the 

 sternum and skull the chief characters are the possession of short and 

 equal pentadactyle lindjs, with never more than three phalanges to 

 a digit, a complete libula and clavicle, a doubly ossilied coracoid, a 

 heterodont dentition — a combination which, wholl}" or in part, we now 

 associate with the Permian genera Procolophm^ PurHimarus^ and 

 others which might be named, the discovery of which constitutes one 

 of the morphological triumphs of our time. 



Beyond this, it may l)e added concerning the Batrachia, that among 

 living pedate forms the anura have alone retained the pentadactyle 

 state and the complete maxillo-jugal arch, and that the Eastern Tylo- 

 totrlton, in the possession of the latter, becomes the least modified 

 urodele extant. These facts lead to the extraordinar}- conclusion that 

 the living Urodela, while of general lowh" organization, are one and 

 all aberrant; and it is not the least important sequel to this that, despite 

 their total loss of liml)s, the Apoda, in the retention of the dermal 

 armor and other features which might be stated arc the most primitive 

 Batrachia that exist. 



The ])atrachian phalangeal formula 22343 was luitil ((uite recently 

 a difficulty in the determination of the precise zoological position of 

 the class, but it has now been overcome l)y the discovery of a Kera- 

 ter2>eton. in the Irish carboniferous having three phalanges on the second 

 digit of both fore and hind limbs, and by that in the Permian of Saxon}'^ 

 of a most remarkal^le creature, Scleroceplialus^ which, if rightl}^ 

 referred to the Stegocephala, had a head encased, as its name implies, 

 in an armature like that of a fish, and the phalangeal formula of a 

 reptile, 23454:. 



Passing from the ])atrachia to the fishes, we have still to admit a 

 gap, since an intermina])lc discussion on figures and fins lias not nar- 

 rowed it in the least. In compensation for this, however, we have to 

 record within the fish series itself pi'ogress greater, perhaps, than with 

 the higher groups. Certainly is this the case if, as to l)ulk, the litera- 

 ture in syst(»matic sand paheontology Ix^, alone taken into account. 



Of the Dipnoi our knowhHJge is fast becoming comi)lete. AVe kiiow 

 that Lejndosiren forms a ))un-ow, and in consideration of a former 

 monstrous proposal to regard this animal, with its .56 paii-s of ribs. 



