No. 2304. REPTILIAN CHARACTERS IN MAMMALS—WORTMAN. 35 



growth ? One might argue with equal facility that the brain case of 

 all bats, moles, or birds in a given osteological collection is composed 

 of a single bone because it does not show sutures. By the same token, 

 the basal Eocene representatives of any of the great orders of mam- 

 mals lived but yesterday in comparison with the remote time when 

 these changes were inaugurated. In a like manner, as no known 

 Cynodont reptile can by any stretch of the imagination be consid- 

 ered directly ancestral to any group of mammals, no argument based 

 upon their structure is very convincing. 



Turning next to a discussion of the element or elements of the 

 premallear tract of Meckel's cartilage mentioned in the preceding 

 pages, I shall begin by quoting from Parker's description of the 

 third stage of the embryo of Chohepus hojfmanni, in which he makes 

 the following statement: ^ 



The main part of Meckel's cartilage has been used up— partly ossified and lost in 

 the ramus and partly absorbed. The head of the Malleus, the osseous matter of which 

 runs forward as the styliform "processus gracilis," has in front of it yet a large tract of 

 the primary mandible. This thick semiosseous hook curves it^eU, after it becomes 

 detached from the main bar, round the front of the tympanic cavity. The distal tliird 

 is unossified; this bony tract is essentially a second "articulare internum" such as is' 

 seen in Holostean Ganoids. But this tract has a greater interest for the morphologist 

 even than this, for such a remnant of the normal mandible is often present in adult 

 marsupials, and for a time during the first autumn, the mole has a similar malleus, as 

 I shall show in my next part. More than that, in a simil9,r malleus of a young Koala 

 (Phascolarctos cinereus) of the same size nearly as this young Unau, I find two small 

 membrane bones in this premallear tract. 



It seems strange tx) me that this higiily important statement by 

 Parker should have been so completely overlooked by subsequent 

 investigators, since I have been unable to find any reference to it in 

 any later work. In regard to that part of it, however, which speaks 

 of "membrane bones" in the Koala, I have not had any suitable 

 material of this species for study and I am unable to say, therefore, 

 whether or not these elements of the premallear tract are ' 'membrane 

 bones," as he calls them. There can be no question, however, that 

 they are preceded by a cartilaginous mold or matrix, just as much as 

 are the incus and malleus, and whether they ossify wholly from the 

 perichondrial membrane surrounding the cartilage, or whether a por- 

 tion of the cartilage is involved in the process, they are certainly not 

 entitled to the appellation of ' 'mem.brane bones" in the same sense as 

 this term is applied to the frontals, parietals, nasals, or other bones 

 of this category. In the case of Tupaia and many other Insectivora, 

 as well as in SarcopJiilus and BidelpJiis, in which this element per- 

 sists and undergoes ossification, the resulting bone is a solid rod or 

 bar, and as far as I am able to judge, exactly like the stylo-hyal, 

 epi-hyal, or cerato-hyal pieces of the hyo-mandibular arch. 



» Morphology of the Skull, p. 65. 



