46 



SCIENCE. 



[Vol. XX. No. 494^ 



for aa which is found in other combinations where it has 

 the same signification. 



So far I have found no marks indicating the plural ; this 

 may be represented by duplications. 



OSTEOLOGICAL NOTES. 



ET D. D. SLADE. 



The jugal arch is present in all of the order Eodentia, and 

 is generally complete, although it exhibits many modifica- 

 tions in its composition. Three bones form the arch, which 

 is straight or slightly curved horizontally, while it almost 

 invariably presents a curvature downwards. The position 

 of the jugal therein serves as a determining character in 

 grouping the various families of the order. 



The temporal fossa is often little developed, showing fee- 

 ble energy in the action of the temporal muscle. On the 

 contrary, the pterygoid plates and fossge are often largely 

 increased in relation to the enlarged development of the 

 muscular insertions. In close connection with these condi- 

 tions, the coronoid process of the mandible is small, and 

 even rudimentary, wliile the parts about the angle are 

 largely expanded. The condyle is little elevated and pre- 

 sents, with few exceptions, an antero-posterior articulating 

 surface. 



Post-orbital processes of the frontals exist in a few of the 

 families, but there is in no case a corresponding process from 

 the arch. The orbit is never separated from the temporal 

 fossa. 



In many of the rodents there is present a more or less ex- 

 tensive dilatation of the infra-orbital foramen, through 

 which passes, in addition to the nerve, that portion of the 

 masseter muscle which has its insertion upon the maxilla. 

 This extends around the back of the jugal process of the 

 maxilla in a pulley-like manner, to an insertion just below 

 the socket of the mandibular pragmolar, and thus cooperates 

 with the temporal in moving the mandible in a vertical di- 

 rection. This attachment of a head of the masseter is pe- 

 culiar to the order, and explains the use of the vacuity in 

 the maxilla which is oftentimes of vast relative propor- 

 tions. 



Assuming the present classification, all existing Rodentia 

 may be brought into two groups, the Simplicidentata and 

 the Duplicidentata. The first embraces the Sciuromorpha, 

 Hystricomorpha, Myomorpha, and the second, the Lago- 

 morpha. 



In the Sciuromorpha, the jugal forms the greater part of 

 the arch, extending forward to the lacrymal, and posteriorly 

 to the glenoid cavity, of which it forms the outer wall, and 

 it is not supported below by a continuation backwards of 

 the process of the maxilla. In the more typical forms there 

 is no enlargement of the infra-orbital opening, while the 

 post-orbital processes of the frontals are characteristic of the 

 family Sciuridae. The external pterygoid plate is entirely 

 "wanting, and there is no fossa. 



The jugal arch in the Myomorpha is for the most part 

 slender, and the jugal, which does not extend far forward, 

 is supported by the continuation below of the maxillary 

 process. The zygomatic process of the squamosal is short. 

 No post-orbital process of the frontal exists. The infra- 

 orbital opening varies. In the family Muridse, especially 

 in the typical forms, this opening is perpendicular, wide 

 above and narrow below, while the lower root of the 

 zygomatic process of the maxilla is flattened into a thin 

 perpendicular plate. Very much the same condition exists 



in the Myoxidse, while in the Dipodidse the foramen is as 

 large as the orbit, rounded, and has a separate canal for the 

 nerve. The malar ascends to the lacrymal in a flattened 

 plate. In close connections with these conditions the 

 coronoid process of the mandible is small and rudimenary, 

 while the parts around the angle of the ramus are much 

 developed. 



In the Hystricomorpha the arch is stout. The jugal is 

 not supported by the continuation of the maxillary process, 

 and generally does not advance far forward. The infra- 

 orbital vacuity is large, and is either triangular or oval. 

 The coronary process and the condyle are but slightly elevated 

 above the dental series. 



In the Chinchillidse the jugal extends forward to the lac- 

 rymal. In the Dasyproctidse, Ceelogenys is characterized by 

 the extraordinary development of the jugal arch, which 

 presents an enormous vertical curvature, two-thirds of the 

 anterior portion of which, constituting the maxilla, is hol- 

 lowed out into a cavity which communicates with the mouth. 

 The nerve passes through a separate canal, adjacent to the 

 infra-orbital opening. 



In the sub-order Duplicidentata, the jugal arch is well de- 

 veloped. In the family Leporidse there are large wing-like, 

 post-orbital processes, while the jugal, but feebly supported 

 by the maxillary process, continues posteriorly to aid in the 

 formation of the outer side of the glenoid articular surface, 

 passing beneath the process of the squamosal. 



In the Lagomyidae there are no post-orbital processes, and 

 the posterier angle of the jugal is carried backward nearly 

 to the auditory meatus. The infra-orbital opening in the 

 Duplicidentata is of the usual size. The angle of the jaw 

 is rounded and the coronoid process much produced up- 

 wards. 



In considering the significance of the jugal arch in the 

 Rodentia, the peculiar vertical curvature downwards, which 

 has already been noted, and which is a decided manifesta- 

 tion of weakness, must be taken into account. This condi- 

 tion is compensated in some of the families by the unusual 

 arrangement made in the distribution of the muscular inser- 

 tions of the masseter through the infra-orbital opening, by 

 which increased energy is imparted to the powers of masti- 

 cation, and whereby the action of the mandible is rendered 

 fully equal to the demand u~pon its efforts. 



In those families where the above condition does not exist 

 it is evident that the strength of the arch is still sufficient 

 for the antero posterior movement of the articulation so pe- 

 culiar to the Rodentia and so characteristic of the act of 

 gnawing. 



The relation of the arch to the neighboring parts must 

 also be remarked. For example, the ascending ramus of 

 the mandible difi'ers according to the food. Elevated in the 

 Leporidse, it is short in the SciuridEe, and still shorter in the 

 Muridse. 



In the first the coronoid is broad, projects but slightly, is 

 near the condyle, and far distant from the molar series, 

 while the angle of the jaw is broad and well rounded, as in 

 the Lagomyidas. 



In the other two families, squirrels and rats, the coronoid 

 is feeble, pointed, and placed at equal distances between the 

 condyle and the last molar; thus the masseter does not pos- 

 sess a leverage as advantageous as in the hare. This mus- 

 cle, however, in the rats has its maxillary attachments much 

 developed, while few fibres spring from the arch — a condi- 

 tion correlative with the feebleness of this last. 



Cambridge, June 21. 



