371 



the posterior third part of the zygomatic expansion, the rest being formed by the maxillary, 

 which is unusually and enormously developed. The squamosal forms only the base of the 

 zygoma ; it is grooved below for the mandibular joint, to which the malar contributes the 

 outer part. The nasal processes of the premaxillary do not extend so far back as the nasals : 

 the large antorbital vacuity is reduced by the maxillary zygomatic plate to a crescentic form. 



Hunter ian. 



2043. The skull of the Paca (Cadogenys Paca), vertically bisected. 



The zygomatic expansion of the maxillary is deeply excavated on the inner side ; it forms, 

 in the recent animal, a large bony capsule on each side of the mouth, communicating with the 

 mouth and lined by the buccal membrane. The basioccipital, basisphenoid, and presphenoid 

 have coalesced to form a continuous bony floor for the cranial cavity. A vertical sinus ter- 

 minating below in two small foramina, communicatipg with the orbit, divides the rhinence- 

 phalic from the prosencephalic fossa. Bristles are passed through the foramina and also 

 through the branch of the lateral sinus which leads from above the petrosal to between the 

 squamosal and tympanic externally. The olfactory cavity extends backwards beneath the 

 rhinencephalic one, but not above it. The wide aperture of the zygomatic bony cavity is 

 well shown in this section. The third division of the fifth notches the alisphenoid posteriorly, 

 the foramen ovale being an irregular fissure between the ah- and basi-sphenoid and the 

 petrosal. .The ectopterygoid process joins the proper pterygoid, and, with the entopterygoid 

 plate, completes a wide interpterygoid canal. The base of the ectopterygoid is perforated 

 by a wide ectocarotid foramen. 



Presented by Charles Stokes, Esq., F.R.S. 



2044. The skull of a Paca (Cadogenys Paca), in three transverse sections. 



The premolars are still concealed in the substance of the jaw, being the last of the molar 

 series to be developed. In the posterior section may be seen the exclusion of the squamosal 

 from the cranial cavity by a fissure which widens as it descends between the squamosal and 

 petrotympanic. At the back part of the middle section the rest of the venous sinus occupying 

 this fissure is shown. In the cavity of the acoustic bulla are seen the horizontal septum 

 dividing the upper from the lower compartment of the anterior half of the bulla, and the 

 termination of the meatus in the lower compartment. The cochlear cavities are exposed. In 

 the fore-part of the middle section the vertical extent of the singular zygomatic chambers are 

 well displayed, and the commencement of the antorbital canals formed by what are sometimes 

 called the upper and lower branches of the anterior root of the zygoma. The right premolar 

 is exposed in its concealed alveolus. The anterior section shows the descending processes of 

 the nasals, and the anterior turbinals. 



Presented by W. J. Sroderip, Esq., FS.S. 



2045. The left ramus of the lower jaw of the same specimen, with the inner wall of 

 the socket of the concealed premolar removed. 



Presented by W. J. Broderip, Esq., F.R.S. 



3B 2 



