284. DR. H. W. MARETT TIMS ON 
Psittacotherium being not far from, if not on the direct, line of 
ancestry [4]. His arguments are based mainly on the presence 
or absence of the first and third incisors; and the condition 
found in Esthonyx, Psittacothertum, Calamodon, and Tillothercum 
are referred to as evidence in support. An elaborate theory is 
then drawn up to show how the Rodent molars may have been 
produced mechanically from the molars occurring in the above- 
mentioned fossil forms. This theory, though very ingeniously 
worked out, is but a theory, and cannot be admitted as evidence. 
As to the incisors, though I admit that these forms may be so 
arranged that different stages in their reduction may be made 
to appear, and the increase in the size of 7.2 to become evident, 
nevertheless it must not be forgotten that a similar condition is 
to be met with in the Multituberculata, two incisors only being 
characteristically present in the genus Polymastodon, of which 
one is very large and “‘rather slender, sharply grooved, restricted 
enamel-band, and a deep postero-external groove. The lateral 
incisor [2.8] is a very small conical tooth, compressed antero- 
posteriorly, with its enamel confined to the anterior surface.” 
Further, in a note (“ Note on the Marsupialia Multitubercu- 
lata”) appended to his paper (Joc. cit.), Cope stated that the 
incisors of the Plagiaulacide, Chirogide, and Polymastodontide 
are similar in structure and functions to those in the Rodentia. 
Osborn and Earle also say (loc. cit.) that the condyle of the 
lower jaw is “oval, and its long axis is placed obliquely, not 
antero-posteriorly as in the Rodents.” Cope refers to this 
latter fact as an objection; but it appears to me to be only 
an objection to his “ mechanical theory,” and not to the multi- 
tuberculate theory of descent of the Rodents, for, according to 
Osboru and Earle (Joc. cit.), the obliquity is ‘greater in some 
specimens than in others,” which shows that this is a character 
which is not stable but undergoing modification. 
A further objection may be cited from the joint paper of these 
authors, as they say that Polymastodon foliatus is the most 
primitive type of the genus, being “ distinguished by small size 
and very few tubercles.” With regard to size, I do not think it 
is necessarily any proof of primitiveness ; and as to the number 
of tubercles, surely the statement partakes somewhat of the 
nature of “ begging the question.” 
To sum up the matter, it appears to me that the balance of 
