1895.] t)0 [Slade. 



the length of the squamosal process and that of the maxilla being nearly 

 equal on either side. The temporal and orbital fossie form one depression 

 in the side of the skull. The rami of the mandible are slender and straight 

 and without teeth, angle, or coronoid process. The condj'le is not raised 

 above the level of the remainder of the ramus. 



In the Orycteropidse, the arch is complete, and the horizontal curvature 

 is very slight. The postorbital process is well-developed. The mandible 

 rises high posteriorly, with a coronoid slightly recurved, and with an 

 ascending pointed process on the angular edge below the condyle. 



In the Marsupialia, the jugal arch is always complete, and composed of 

 the jugal, resting on the maxilla and squamosal, the first extending from 

 the lachrymal anteriorly to the glenoid fossa posteriorly, of which it forms 

 the external wall. The process of the squamosal passes above the jugal, 

 being united to it by an almost horizontal suture. The horizontal and 

 vertical curvatures of the arch are considerable, and the space for both 

 temporal and masseter muscular insertions is extensive. The various 

 ridges and crests are large, especially in the Dasyuridaj and Didelphida;. 

 The postorbital of the frontal is present as a rule, although in most forms 

 inconsiderably developed. The ascending ramus of the mandible is less 

 elevated than in several of the orders of the Mammalia. The condyle is 

 but little raised above the molar series. The masseteric fossa is extremely 

 projected at its lower external border, and the mandible, with one excep- 

 tion, has an inverted border to the angle. 



In the Monetremata, the Echidnidse possess an arch in wiiicli the squa- 

 mosal is compressed, and sends forward a slender straight process to join 

 the corresponding slight shaft-like process of the jugal. The horizontal 

 curvature is extremely small. 



In the Ornithorynchidse, the arch is made up of the malar resting upon 

 a process of the maxilla, which, passing straight backwards, unites with 

 the squamosal process that rises far back on the sides of the cranium. 

 While the mandible of the Echidna has but the rudiments of the parts 

 which usually enter into its formation, that of the Ornithorynchus is 

 more fully developed in relation to the attachment of the horny teeth. 



In studying the significance of the jugal arch according as this portion 

 of the mammalian cranium has been presented to us in the preceding 

 pages, while there are modifications in certain groups which are somewhat 

 difficult of explanation, we shall find that the general laws which govern 

 its morphology may be satisfactorily determined. These laws, concisely- 

 speaking, are, that the development of the arch, as shown by the number 

 of the bones, by the degree and the ntimber of the curvatures, by its rela- 

 tion to the orbit and articulation of the mandible, as well as to other 

 neighboring parts, and by the amount of surface presented for muscular 

 insertion, all depend upon the energy and character of the masticatory 

 muscles. That these in turn depend upon, and are closely correlated with 

 the habits and environment of the animal. 



The above laws are very clearly exhibited in the Carnivcra and in the 



PROC. AMER. PHILOS. SOC. XXXIV. 147. I. PEIKTED MAY 9, 1895. 



