OF DIDELPHYS VIRGINIANA. 55 



laterally, the petrosal and squamosal (which separate it from the mastoid), and sometimes 

 the malar : on either side, the parietal and frontal : before, the vomer, ethmo-turbinal, 

 palatine, and pterygoid. There is no actual connection with the tympanic; the mastoid 

 is still further removed. I have never found it confluent with either of these con- 

 tiguous bones : its connection Avith the palatines is perhaps most intimate ; with the 

 pterygoids and petrosals loosest. Its frontal and squamosal sutures are the most exten- 

 sive ; but that with the parietal is of considerable length, since the squamosal does not lift 

 the parietal far away from the sphenoidal ala — though it is difficult to say how much of the 

 spheno-parietal suture is formed by the orbitosphenoid, and how much, if any, represents 

 the union of the neural spine with its proper neurapophysis, the alisphenoid. The basi- 

 sphenoid alone unites with the basioccipital : the alisphenoid alone with the petrosal and 

 squamosal ; the orbitosphenoid with the frontal. Junction with the palatines is through 

 three of the elements of the sphenoid : the ali-, orbito- and pre-sphenoid ; union with the 

 former being effected by the " pterapophyses " to be presently noticed : the sphenoidal 

 relations of the pterygoids are almost wholly brought about by the same means. The 

 articulation of the sphenoid with the basioccipital and vomer respectively is simple abut>- 

 ment of the thickened, squarely truncated extremities of these bones ; that of the palar- 

 tines and pterygoids is the lateral apposition of elongated processes; that with squamosal, 

 parietal and frontal is the true, squamous suture, most decided in the latter case, where 

 the orbitosphenoid very extensively overlaps. When the alisphenoid reaches the malar 

 at all, it is a simple touching of the tip of its slender, lateral, articular process. Junction 

 with the petrosals is imperfect and irregular. 



The compound " sphenoid bone," thus made up and related, presents a stout, flattened 

 "body," tapering from behind forward, distinguishable from the "wings" by its shape and 

 solidity, and separated from them by grooves, ridges and foramina, apparent both above 

 and below. The under surface is smooth ; on either side are seen, from behind forward, 

 the termination of the foramen lacerum, medium : two foramina looking forward and out- 

 ward, separated by a slight bony bridge, the posterior opening leading backward by a groove 

 into the foramen lacerum, and forward into the sella turcica : the antei'ior, which is the 

 larger, curving inward and communicating with its fellow below the sella : the groove sep- 

 arating the pterapophyses completes the lateral boundary of the body. Inside, the body 

 presents a deep hemispherical or hemiellipsoidal excavation, the sella turcica, on either 

 side of which (but really in the ala of the bone) is the entrance of the foramen rotundum, 

 now a long canal, that opens just outside the root of the pterapophysis. More anteriorly 

 the body is bounded on either side by a deep semicylindrical canal, which conducts the 

 optic nerves out through the foramen lacerum anterius. 



The alisphenoidal part of the bone stands away from the body, on nearly the same 

 horizontal plane, having but little upward inclination ; and supports three very noticeable 

 processes. The most posterior of these is a moderately large bulla ossea, or conch, in sub- 

 serviency to the acoustic organ. It is a hemiellipsoid in shape, its convexity looking for- 

 ward and downward, its large opening directly towards the tympanum. It is small in size 

 compared with that occurring in some marsupials. Besides its physiological relation to the 

 organ of hearing, it serves to- complete, internally, the irregular parapet surrounding the 

 fossa in which the delicate tympanic hangs suspended : the other minarets being the par- 

 occipital, mastoid and zygomatic process of the squamosal. The petrosal lies along its 



