TRANSACTIONS OF SECTION D. 625 



he suspect the rest. It is, however, a most intensely interesting fact that, whereas 

 by a Huxleian triumph the vertebral theory of the skull was overthrown, in these 

 later Huxleian days the proof of the incorporation of a portion of the vertebral 

 region of the trunk into the mammalian occiput should have marked the 

 succeeding epoch in advance. The existence of twelve pairs of cranial nerves 

 which all the Amniota possess involves them in this change ; and the fact that in 

 all Batrachia there are but ten, enables us to draw a hard and fast line between 

 batrachian and amniote series. 



It may be urged, as an objection, that since we have long been familiar with a 

 fusion of vertebrae and skull in various piscine forms, the force of this distinction 

 is weakened. But this cannot be ; since, in respect to the investing sheaths and 

 processes of development which lie at the root of the genesis of the vertebral 

 skeleton, the fishes stand distinct from the Batrachia and Amniota, which are 

 agreed.'- So forcible is this consideration that it behoves us to express it in 

 words, and I have elsewhere proposed to discriminate between the series of 

 terrestrial Vertebrata as arch<e- and sijn-craniate.''^ 



Similarly there is no proof that any batrachian, living or extinct (and in this 

 I include the Stegocephala as a whole), possesses a costal sternum. So far as their 

 development is known, the cartilages in these animals called ' sternal ' are either 

 coracoidal or sui generis?'' The costal sternum, like the syncraniate skull, is dis- 

 tinctive of the Amniota alone. Had the Stegocephala possessed it even in carti- 

 lage, there is reason to think it might have been preserved, as it has been in the 

 colossal Mososaur Tylosmirus of the American Cretaceous.'^ When to this it is 

 added that whereas, in the presence of a costal sternum, the mechanism of inflation 

 pf the lung involves the body-wall, in its absence it mainly involves the mouhh (as 

 in all fishes and balrachians), the hard and sharp line between the Batrachia and 

 Amniota may be expressed by the formula that the former are archcecraniate and 

 stomatophysous, the latter syncraniate and smnatophysous. 



There are allied topics which might be considered did our time permit ; but 

 one certain outcome of this is that there is an end to the notion of a batrachian 

 ancestry for the Mammalia. And when, on this basis, we sum up the characters 

 demanded of the stock from which the Mammalia have been derived, we find them 

 to be precisely those occurring outside the Mammalia in the Anomodont Eeptiles 

 alone. Beyond the sternum and skull, the chief characters are the possession of 

 short and equal pentadactyle limbs, with never more than three phalanges to a 

 digit, a complete fibula and clavicle, a doubly ossified coracoid, a heterodont 

 dentition — a combination which, wholly or in part, we now associate with the 

 Permian genera Procolophon, Pariasnurus, and others which might be named, the 

 discovery of which constitutes one of the morphological triumphs of our time.'" 



Beyond this, it may be added, concerning the Batrachia, that among living 

 pedate forms the Anura have alone retained the pentadactyle state and the com- 

 plete maxillo-jugal arch, and that the Eastern Tylototriton, in the possession of 

 the latter, becomes the least modifled urodele extant." These facts lead to the 

 extraordinary conclusion that the living Urodela, while of general lowly organisa- 

 tion, are one and all aberrant ; and it is not the least important sequel to this 

 that, despite their total loss of limbs, the Apodn, in the retention of the dermal 

 armour and other features which might be stated are the most primitive Batrachia 

 that exist.'* 



The batrachian phalangeal formula 2234.3 was until quite recently a difficulty 

 in the determination of the precise zoological position of the class ; but it has now- 

 been overcome, by the discovery of a Keraterpeton in the Irish Carboniferous 

 having three phalanges on the second digit of both fore and hind limbs,'^ and by 

 that in_ the Permian of Saxony of a most remarkable creature, Sclerocephalus, 

 which, if rightly 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 Batrachia to the Fishes, we have still to admit a gap, since an 

 interminable discussion on fingers and fins has not narrowed it in the least. In 

 compensation for this, however, we have to lecord within the fish series itself 

 1902. s s 



