633 
the osteology of HYiENODON. 
VIII. THE UELATIONSHIPS OF HY^NODON. 
It has been customary of late to include Hycenodon and Pterodon with 
Tontr. Xo"?) Tl^ " ^ «« the creo- 
donts (.No. 0. The propriety ot such reference will depend upon the significance 
.dnch IS assigned to the tainily groups. In the classification of recent anhnals the 
family is einployed to include all the genera which agree with one another in the 
possession of certain dehnite structural characters, and it thus often embraces mem- 
bers ot many collateral lines. This principle is useful in grouping the recent forms 
and gives e.xpressioii to the relationships of animals existing together at any mven 
period ot tl.e earth’s history. In the plan of fomily classification adopted by Os- 
born and bchlosser, on the other band, the tainily represents a single phylogenetic 
hue or branch, which may include short side-branches not leading to permanent 
modifications. This method is useful to express the relationships obtaining between 
the various succes,sive faunas which have been discovered. Each of these methods 
thus possesses certain advantages and each is exposed to the necessity of more or 
less arbitrarily separating allied genera. Schlosser’s method, however, is much 
better adapted to the needs of paleontological inquiry; indeed, it is almost impos- 
sible to use the other with any degree of satisfaction. 
Thanks to the researches of Osborn (No. 3) and Wortman (No. 8) the struc- 
ture of Oxycena and Patriofelis is now well understood and a detailed comparison 
ot these genera with IlycPfwdon may be made. As the result of such comparison 
I have no hesitation in adojiting Wortman’s suggestion of a return to Cope’s origi- 
nal scheme and separating the Oxyanidce from the Hyce7iodontid(e . That these 
two families are nearly' related to each other and were derived from a common stock 
is clear, hut with all their resemblances, they represent diverging lines. In the 
Oxycenidtr the face is much shortened, with a consequent reduction in the number 
of teeth. This reduction affects the molars principally, the formula being m^, but 
in effect is almost lost, as well ; it is very small and forms a transversely directed 
tubercle, while is much the largest of all the cheek-teeth. The fourth upper 
preinolar is a well-developed sectorial, with large posterior cutting edge, or tritocone, 
and in .some species with antero-external cusps. In the lower jaw mi is large, 
though somewhat smaller than ni 2 - which with forms the principal pair of sec- 
torial teeth, while Jha has disappeared. In Patriofelis the reduction of teeth hag 
proceeded still farther .and gives the formula, ™ both genera there are 
but two pairs of lower incisors. In the Hyceiwdontidce the emphasis of develop- 
ment, so to speak, is differently placed, the principal pair of sectorials being m2 and 
ni 3 , which are much the largest teeth in their respective series; a second and less 
important pair is formed by' and ni2. while ml is very greatly reduced in size and 
smaller than most of the preinohars. The last upper premolar is not sectorial in 
form, the tritocone being but little developed. 
,Si„,il„r divergences .appenr in the skull. The face in the Oxy^md^ ,s short 
•and abruptly truncated ; the zygomatic arches are enormously developed and curve 
