Slade.] ^^ [March 15, 



margins rise above the level of the cranium, giving additional origin to 

 the large temporal muscles." There is no postorbital process given oflF 

 either by the frontal or zygomatic arch. As regards the mandible, the 

 coronoid process is little elevated, and in some species is nearly level with 

 the transversely extended condyle. 



In the third group the arch is incomplete, and in one instance, at least, 

 may be described as entirely absent. In the CentetidsE, the skull is long 

 and narrow, and marked by largely developed occipital and sagittal 

 crests which serve as attachments for muscles of temporal origin. The 

 zygomatic processes of the maxilla and squamosal are very short and rudi- 

 mentary, while the malar is entirely absent. The temporal fossae are very 

 large, and the skull retains nearly the same width at their anterior and 

 posterior regions. There is not a trace of a postorbital process. The 

 infraorbital foramen is circular and capacious. There are no pterygoid 

 fossae. The coronoid process of the mandible is largely developed, its 

 inner surface being concave, and its outer surface flattened. The condyle 

 is small and circular, while the glenoid surface is transversely concave. 



The other families of this group, with the exception of the Soricidae, agree 

 with the Centetidoe in the modifications of the skull that have been 

 described. In the Soricidae, the cranium is broadest just behind the glen- 

 oid surfaces. There is no jugal arch and no trace of a postorbital pro- 

 cess. Frequently there is present a strongly marked lamboidal ridge as 

 well as a sagittal crest. There is no pterygoid fossa, but very large vacui- 

 ties exist on each side of the basis cranil. The mandible resembles that of 

 the Talpidae, although the horizontal ramus is shorter, while the ascend- 

 ing one " presents a very large and singularly deep excavation upon its 

 internal surface quite characteristic of the genus." The articular surface 

 of the condyle looks backwards instead of upwards. The angle of the 

 jaw is elongated and thin. The infraorbital is large and bounded pos- 

 teriorly by an osseous bar. 



The jugal arch in the Rodentia is always present, and is generally com- 

 plete, although it exhibits many modifications in its composition. Three 

 bones form the arch, which is straight or slightl}'^ 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 small, showing feeble energy in the action 

 of the temporal muscle. On the contrary, the pterygoid plates and fossae 

 are often largely increased in relation to the enlarged development of the 

 muscular insertions. In close connection with these conditions, the coro- 

 noid process of the mandible is small and even rudimentary, while the 

 parts about the angle are largely expanded. The condyle is little elevated 

 and presents, with few exceptions, an anteroposterior articulating surface. 

 Postorbital processes of the frontals exist in a few of the families, but in 

 no case is there a corresponding process from the arch. The orbit is never 

 separated from the temporal fossa. 



