2^2 OS THE STRUCTURE AND CLASSIFICATION 
this material, and have tlms been obliged to leave several mooted points in doubt. 
There are some discrepancies between the earlier and later pages of this memoir, 
owing to its having occupied nearly nine months in passing through the press. This 
has been in spite of the kind efforts of Dr. Edward J. Nolan, Secretary of the 
Academy, to hurry the matter forward. The delay has, however, given me the 
benefit, in the latter portion of the work, of the valuable recent contributions of 
Lvdekkcr, Oldfield Thomas, Schlosser and Cope upon this and related subjects. 
J'Bixarri>s, July. 1S88. 
APPENDIX. 
1. Tiik .MynoiiYoii) (troove in the Mesozoic and Recent Mammalia. Professor 
Owcm descrilied and figured a mylohyoid groove in Myrmecohim and, as it is univer- 
sally present in the Mesozoic mammals of the Second Group, much stress has been 
laid upon it in classification. Ur. Otto Meyer recently called the writer’s attention 
to the fact that the groove in the Myrmecohim jaw is not similar to that in the 
Mesozoic mammals, and in any case questioned its homology and taxonomic im- 
IMirtance. 'I'his led me to examine the lower jaws of all the marsupials and 
primates in the collections of Princeton, the Philadelphia Academy and Yale College, 
with the following results: 1° A groove similar to the mylohyoid of the human 
jaw is Irecpicntly, but not con.stantly, present among the primates ; in Gorilla, 
strongly developeil ; IVoglodytes, wanting ; Simla, faintly developed ; Gynocephulm, 
very distinct. It varies with age and somewhat within the species. 2° Among the 
Marsupials this groove is even more variable, never very distinct, constantly subject 
to individual variation : Myrmecohim, wanting in the two specimens in the Yale 
Mu«-um, also in the numerous specimens in the British Museum, as kindly observed 
for me bj .Mr. Oldfield Thomas ; Phascolomys, present in only one-half the specimens 
e.xamine<l : faintly seen in some specimens of DidelpJiys and Dasypm ; Dasyurus, 
nylacmw and Itetiongia, absent in all the specimens examined. 3° In all the above 
rases this groove extends obliquely downwards and forwards from the orifice of the 
^tal < anal . there is thus little doubt that it lodges the mylohyoid nerve or artery, 
which brancii from the dental pair at this orifice. 4° In all the Mesozoic mammals, 
m wJuch the groove has been observed, it invariably extends from near the dental 
toramen for a greater or less distance along the inner face of the ramus, sometimes 
n-m h border {PlLmcohtlterium) , sometimes reaching the 
/, A . ' "^‘01 termm). hrom its constant relation to the dental canal and varia- 
ner,-e lr T' T the mylohyoid 
“y “aer supposition. 6» oino- 
. . le anterior liorder of the pterygoid fossa is not clearly defined as in 
