1884.] 45 ICope. 



with the Mammalia, two are of first class importance (Nos. 1 and 5); three 

 are of great but unascertained degree of importance (Nos. 4, 6 and 8), 

 and one (No. 9) is of less importance. The two characters (Nos. 2 and 5) 

 in which the Mammalia agree with the Batrachia, are of high importance, 

 but one of them is also a point in which the Pelycosauria agree with both 

 (structure of the coracoid bone, No. 5). There is but one character, the 

 distinctness of the quadrate bone, in which the Batrachia agree with 

 the Reptilia in general. 



The preceding comparison renders it extremely probable that the 

 Mammalia are descended from the Pelycosaurian Reptilia. The usual 

 definitions have been invalidated, excepting that of the occipital con- 

 dyles, but even this is not so absolute a character as has been supposed. 

 In the gecko lizard, Uroplates, the occipital condyle is represented 

 by the exoccipital pieces only, the basioccipital element being omitted 

 nearly as in the Mammalia. Professors Huxley and Parker have declared 

 it as most probable that the true ancestor of the Mammalia have been the 

 Batrachia. It is evident that the Pelycosauria are in various respects the 

 most Batrachian of the Beptilia, for they agree with them in three and 

 parts of two other characters of the nine above enumerated. One of the 

 latter is the structure of the posterior foot, which displays much less 

 modification from the Batrachian type than that of the ordinary Reptilia. 



The first evidence of the resemblance of the Pelycosauria to the Mam- 

 malia was empirical and not conclusive. This consisted in the characters 

 derived from the long bones of the limbs. Professor Owen first called at- 

 tention to this resemblance in the genus Cynodraco, which is a Theromorph 

 reptile. I next pointed out corresponding peculiarities in the humeri of the 

 American Therornorphs. I subsequently showed the resemblance between 

 the pelvis of the Pelycosaur division, and that of the Monotremata. This 

 was followed by a demonstration of the resemblance between the coracoid 

 of the Pelycosauria and the Mammalia of the Monotrematous order, 

 especially the family of the Platypodidse. The present article now adds 

 that the structure of the posterior foot approaches near to that of the 

 Monotremata ; and that the os quadratum and tbe ribs are essentially like 

 the corresponding parts in all the Mammalia. The last three points are 

 essential and fundamental. The three great distinctions between the 

 Mammalia and Reptilia in the skeleton are : (1) in the quadrate bone ; 

 (2) in the coracoid bone, and (3) in the occipital condyle. Of these the 

 last only now remains, and this is weakened by the presence of the Mam- 

 malian type in the geckotian lizard already referred to. The only inter- 

 ruption in the series which has not yet been overcome is in the columella 

 auris. No reptile is yet known where that element is divided into incus, 

 orbicularis, and stapes, as in the Mammalia and some Batrachia (according 

 to Albrecht). Of course the above comparison with the Monotremata con- 

 siders the latter order in its proper ordinal definitions, and not in its special 

 subordinate modifications now existing, the Platytidse and Tachyglos- 

 sidse. Montremata dentition like that of the known Jurassic and Triassic 

 Mammalia will doubtless yet be discovered in beds of those ages. 



