16 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 51 , 



^U47.^. 



Mcr//. 



spicule. This may be the remains of the enlarged portion of these 

 elements figured by Parker. 



Sirenia. — The American sea cow shows at least three elements in 

 the jugal arch in tlie adult skull— namely, a quadrato-jugal piece, 

 articulating with the zj^gomatic process of the squamosal, a jugal 

 element underlying the orbit, and a post orbital piece which forms 

 the postorbital process of the arcli. There are, in addition to these, 

 in some specimens, a small ossicle developed just above the vestigial 

 lachrymal and a pair of ossicles lying below and upon the outer side 

 of the reduced nasals. As there is no postorbital process of either 

 frontal or parietal, no trace of a separate element in this situation 

 is fomid. In all of the specimens which I have thus far examined I 

 have been unable to detect the presence of an element associated 

 with the tympanic, which corresponds to that aheady described in 



Tupaia, Gijmura, and others, 

 although from certain appear- 

 ances of the dried skull, it is not 

 altogether unlikely that care- 

 fully prepared younger speci- 

 mens maj'- show its presence. 



Ungulata. — Among the Un- 

 gulates the horses show the 

 presence of at least two ele- 

 ments composing the malar in 

 the younger stages, one of 

 which represents the jugal and 

 the other the quadratojugal. 

 It is more than likely also that 

 the bony bar bounding the pos- 

 terior part of the orbit is com- 

 posed of an upper and lower element in the young stages, although 

 I have not seen tlie evidence to confirm this. 



Rodentia. — The chief point of interest in this group as far as the 

 presence of these reptilian elements is concerned relates for the most 

 part to the composition of the zygomatic arch. There is considerable 

 evidence in botli the Hystricoidae and the Sciuroidae that there were 

 originally at least two elements entering into the arch, namely, a 

 jugal which has disappeared in many forms and the quadrato-jugal, 

 which now constitutes the principal remaining piece. In the squir- 

 rels that part of the long malar which runs forward to the lachrymal 

 along the imder edge of the orbit is sometimes cut off b}^ a suture into 

 a distinct bone, as is hkewise the case somewhat more frequently in 

 the Hystricoidae. 



LagomorpJia. — In the Lagomorpha the maxillary seemingly runs 

 backwards behind the glenoid cavity and forms the whole of the zygo- 

 matic arch. Sufficiently young skulls show, however, that this appar- 



iFia. 9.— CHOLOErus hoffmanni. In., incus; Mall., 

 malleus; Man. Mall., manubrium of malleus; Pt 

 gr., PBOCEssus gracilis; Quad., quadrate; Sta.. 

 stapes. 



