OF DIDELPHYS VIRGINIANA. g.! 



goids "repeat the connections they present in birds." In this marsupial, at least, these 

 bones, so far from having any connection with the tympanies, do not reach half way from 

 the palatines. 



The lachrymal is of large size, and forms a considerable part of the orbital wall. While 

 its general shape is that of a thin, flattened, triangular scale, it has one stout process, 

 sticking outward and backward, for articulation with the malar — the point of the process 

 being thrust into a pit in the upper corner of the latter, like a tooth in its socket. The 

 part of the bone that forms the rim of the orbit at the anterior canthus of the eye is thick- 

 ened, and presents a perfectly smooth surface, of a semilunar shape, whose convex border 

 fits the concavity in the margin of the maxilla. The large opening of the lachrymal duct 

 appears in the centre of this surface, leading forward into a canal that runs the whole length 

 of the bone before terminating in the nasal chamber. This foramen appears just outside the 

 orbital rim — upon the face, rather than in the orbit itself. With the exception of the malar 

 articulation above described, all the sutures are squamous ; the edge of the bone overlaps 

 both frontal and palatine, but is overlapped by the maxillary, and this to such an extent, 

 that only its thickened central portion appears upon the outside. The inner surface forms 

 part of the lateral wall of the nasal cavity ; it is a little concave, and slightly impressed 

 with a few irregular ridges and depressions. 



The vomer and ethmoid (ethmo-turbinal — olfactory sense-capsule) may be most con- 

 veniently noticed together, as they form virtually one bone. In superficial extent it is 

 one of the largest bones of the head ; but it is very light and delicate in texture, the 

 spongy masses forming by far the greater part. In shape it corresponds, in a general way, 

 to the cavity of the nasal chamber, being something of a parallelopipedon, but narrowing 

 supero-inferiorly from behind forward ; similarly wedge-shaped transversely from before 

 backward, and with a deep excavation at its upper posterior corner ; thus resembling, except 

 in the latter particular, the frustrum of a double wedge. It consists of a sufficiently strong 

 vertical median plate (perpendicular ethmoidal lamma= coalesced prefrontals?) straight 

 edged above and below, nearly vertically truncated anteriorly, and bearing upon either side 

 the spongy convolutions, and bounded below, as by a horizontal keel, by the long, slender 

 vomer, which coossifies throughout nearly all its length. The posterior lower corner is the 

 only point where the bone acquires any decided solidity. At this point the bone runs out 

 into a knob-like process, with vertical sides, convex base, and slightly concave superior sur- 

 face ; this process is received in, and exactly fits, the interspace between the anterior mar- 

 gins of the orbito-sphenoids, thus projecting into the cranial chamber and forming a protu- 

 berance, just behind the cribriform plate, upon which the rhinencephalon rests ; it also 

 arches over, and forms the roof of the passage over the presphenoid, converting the 

 grooves on either side of this bone, by which the optic nerves escape from the cranium, into 

 canals ; behind, these canals inter-communicate, there being no median septum ; but ante- 

 riorly they are separated by the articulation of the vomer with the presphenoid. This 

 process is thus hidden within the cavity of the cranium ; but the knob that is the truncated 

 base of the vomer, and is borne upon its under surface, appears, upon viewing the outside of 

 the skull from below, as a smooth bone, lying deep between the spheno-pterapophyses, 

 abutting against the end of the presphenoid, and in direct forward continuation of the latter 

 into the nasal cavity, forming the roof of the posterior nares. Further on the bone passes 

 between, and is concealed by, the palatals — a faintly marked groove on either side of the 



UKUOIRS HOST. 800. NAT. HIST. VOL. II. 16 



