654 NEW FOSSIL MAMMALIA 



i. e., the excessive development of the pre-molars and the reduced num- 

 ber and size of the true molars. 



" The condyle of Plagiaulax, therefore," observes Falconer, " incul- 

 cates an emphatic warning against too much stress being laid upon any 

 single character in Palseontological determinations." And he adds that 

 " this ancient fossil is interesting not only for its affinity to the existing 

 Kangaroo-rat of Australia, but also as seeming to furnish a crucial test 

 of the soundness, in some respects, of certain generalizations which have 

 been put forward respecting the order of the successive appearance of 

 mammalia upon the surface of the earth. It is maintained by some 

 British palaeontologists and comparative physiologists of high authority, 

 that, while there is no positive proof of serial progressive development 

 from the lower to the higher forms, there is clear evidence of another 

 order of development or passage, viz., from the general to the special, as 

 we pass from the oldest tertiary to the modern period. It is urged by 

 the advocates of this doctrine, that the mammalia of the Eocene Period 

 assimilated more to the general archetype and embryonic condition of 

 Vertebrate organization, while the mammalia of later times successively 

 furnish examples of increasing deviation from the original or normal 

 type as well as of special adaptation. Among other arguments, they in- 

 sist that the earliest Eocene mammalia, both herbivorous and carnivo- 

 rous, possessed in most cases the full complement of teeth ; while forms 

 characteristic of later times, such as the Pelidos and Ruminantia, are 

 remarkable for special suppression of these organs. If the generalization 

 were really of as wide an application as has been claimed for it, we ought, 

 in every great family of the mammalia, to find evidence of closer adher- 

 ence to the archetype the further we recede in time. But so far is this 

 from being the case, that Plagiaulax, the oldest well-ascertained herbiv- 

 orous mammal, presents to us the most special exception to be met 

 with in the whole range of marsupialia, fossil or recent. It had the 

 smallest number of true molars of any known genus in that sub-class ; 

 thus exhibiting at the most distant end of the chain the very characters 

 which, under the influence of the assumed law, we ought only to have 

 found in tome type of existing marsupials." 



While the MS. of these pages was preparing for the press (February 

 10, 1857), part of the cranium of a mammal was received from Mr. 

 Beckles, comprising the two superior maxillary bones and teeth, with 

 the intermediate palate crushed, 6f a small insectivore. On the right 

 side of the jaw the whole series of molar teeth and the incisors are seen. 

 The grinders are more numerous, but the dental characters, says Dr. 

 Falconer, bear a relation to those of the insectivorous genus Ericulus, 

 peculiar to Madagascar, and from the general bearing of the evidence, 

 it is presumed that the fossil was a minute Placental Insectivore.* 



° Although the teeth differ considerably in shape from those of the other 

 Purbeck fossils, it is just possible that this creature may be the same as some 

 of the minuter species above alluded to, and known as yet only by their lower 

 jaws. 



