67 



It was afterwards lost far back in the Mammalian series. It 

 is, I think, more probable that the first Mammalia and first 

 Sauropsida diverged from a common ancestral stock in 

 which the ovum had much food-yolk, than that the Mammals 

 were derived from some group of the Sauropsida. The 

 arrangement shewn in the table, besides taking account of 

 the condition of the ovum in the Monotremata, also explains 

 the very decided Amphibian characteristics of the Mammalia 

 pointed out by Huxley,* such as the condition of the pelvis, 

 the skull, and some other parts of the skeleton, if these 

 Amphibian characters can be regarded, as I think they 

 may, as being found in the Proto-Pentadactyloidei, from 

 which both Amphibia and Proto-Amniota are derived. If 

 this view is correct, then the features in which Mammalia 

 resemble Amphibia are really ancestral Proto-Pentadacty- 

 loidean characteristics, and any in which they agree with 

 Sauropsida only were acquired later by the Proto-Amniota. 

 The ancestral Mammals, — which must have developed 

 mammary glands for the nourishment of the young, while the 

 epidermic exoskeleton assumed the form of hair, and the left 

 fourth aortic arch became the chief artery leaving the heart, 

 — are most nearly represented at the present day by the 

 Monotremata, a group in which the brain and some parts of 

 the skeleton exhibit a lower stage of organisation than is 

 found in other Mammalia. The oviducts also remain in the 

 primitive condition of two tubes opening independently into 

 a cloaeal cavity, and the mammary glands are not well devel- 

 oped. The few existing forms belonging to the Monotremata 

 are probably the representatives of a group which diverged 

 from the earliest Mammals, and to which Huxley f has given 

 the name Prototheria. The present Monotremata are doubt- 

 less somewhat modified, but retain the characters of the 



* Proceedings Roy. Soc, v. xxviii, p. 395 (1879). 

 t Proceedings Zool. Soc, for 1880, p. 649. 



