AT^SINOITIIEEIUM. 
9 
ave somewhat triaiigolav in section : the sharp anterior angle is continued down- 
wards to the border of the nasal opening (PI. I. and PL IT. tig. 1 a). 'J'lie 
inner face is nearly fiat, the posterior is also flattened, while the outer side 
is rounded. In what may be regarded as an adult female skull the horns are 
much smaller, more erect, and very irregularly ossified at their tips, which are 
rounded anteriorly, but terminate posteriorly in an irregular mass of bone. This 
skull was figured by Mr. Beadnell in his original paper (pis. iii.-v.). The 
specimen in question is fully adult, for the last molar is well worn, all the 
sutures are closed, and the prenasal bar fully ossified. In the young the horns 
are rounded at the summit, where the bone is of a peculiarly fibrous or spongy 
texture, showing that growth Avas actively proceeding at this point. The central 
portion of the horns is occupied by an enormous sinus, the walls being relatively 
thin, about '5 cm. to 1 cm. in thickness in the adult, and much less in the young. 
The sinus occupies the bodies of the nasals and frontals and in the latter extends 
into the small supraorbital horns. Furthermore, it extends back into the parietal 
and in the old animal even into the base of the lateral prominences of the lambdoidal 
crest. In the cranial region the sinus is more or less completely divided into a 
right and left half by a median septum occupying the position of the suture between 
the parietals. Flere the lower table of bone forming the actual wall of the brain- 
case is very thin, while the upper table is greatly thickened. In the frontal and 
nasal regions the separation into tw’o halves is not complete, but there is both in 
the mid-dorsal and mid-ventral line a greatly thickened ridge projecting into the 
cavity of the sinus ; the ventral ridge is the stronger and forms a laterally 
compressed bar of bone. These main longitudinal thickenings are connected 
transversely by less prominent, more or less regularly arranged thickenings of 
the walls, and these again are occasionally united by irregular ridges. In the 
paired portion of the horns there are only transverse ridges running round the 
cavity, with some irregular secondary ridges joining them at a few points. The 
great anterior horns thus consist of comparatively thin bony walls, which are 
strengthened by a complex system of buttresses, and are further braced against 
thrusts in the direction of their long axis by the columns resulting from the union, 
of the nasals and maxillm above noticed, and, in the adult, by the prenasal bar. 
The surface of the horns is marked by a series of shalloAV grooves running mon* 
or less in the direction of their long axis and frequently anastomosing. On the 
antero-lateral faces of the horns, a little above the nasal opening, there are impressions 
of blood-vessels (see PI. I., h.v.') similar to those described on the frontals. 
The ‘pfemaxillm {pinx.) are relatively small bones. The alveolar bolder of each, 
as usual, bears three incisor alveoli. Of these the first pair is considerably the 
largest ; they are rounded with a raised border and are separated in the middle 
line by an interval of about 2*5 cm., in which the edge of the bone is rounded and 
c 
