DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
247 
narrow neck; these parts are all in a continuous tract of cartilage. But from the 
middle of that narrow neck up to the part under the triradiate suture formed by the 
nasals, premaxillaries, and maxillaries (compare figs. 1 and 4), each nasal tube is 
segmented into narrow rings, all of which, except those at the end (fig. 5) are 
perfect below. Yet at the mid-line each ring loses its distinctness, being confluent, 
above and below, with the septum. 
This is a true proboscis , quite like that of an Elephant, and like that of the 
Myxinoid Fishes, except that it is double instead of being single, as in those 
permanent larvce, with their primordial “ cribriform plate.” A little in front of the 
part which swells with the middle turbinals, these rings cease; there are thirty of 
them on each side. 
The ossification of the proper cranium is very abruptly marked off from the 
unossified olfactory labyrinth. The anterior sphenoid is but little less than the 
posterior; it is in three “ centres,” but little united as yet, for the presphenoid ( p.s.) 
is here, as in Sorex and the Marsupials, as distinct an element as the basisphenoid 
( b.s .); it is of the same length and not much narrower, but is somewhat hour-glass 
shaped. The orbitosphenoids have a large and pedate downgrowth proximally (fig. 5, 
o.s.), which fits close to the corresponding postero-lateral vomer ( v".) by the fore part 
of its pedate process. 
The orbitosphenoids lie well over the hind part of the olfactory fossae and their 
cribriform floor ( crp .), and form a concave margin to that part, for they project far 
beyond their basal piece, fore and aft; most behind. Towards each other, right and 
left of the median ethmoidal wall-top, they are rounded and notched, and behind their 
junction with the presphenoid they have a large notch—a quadrant. Their outer margin, 
directed backwards outwards and upwards, is crenate, and the hind margin is also 
sinuous; its inner part ending as a spur just inside the foremost angle of the alisphe- 
noid ( al.s .). Hence these alee do not, with their own basal piece, finish the sphenoidal 
fissure; ” it is completed by the posterior sphenoid, base and wing, with the help 
of the pre-basisphenoidal synchondrosis. The widened hind part of the presphenoid 
runs into the large oblique sphenoidal fissure, making it reniform, instead of oval. 
The optic, as well as the orbited, nerves pass through this large, dilated fissure; 
there is no optic foramen ; thus there are three Marsupial characters in this anterior 
sphenoid: (1) the orbito-sphenoid is only slightly overlapped by the alisphenoid— 
there is no gap, only a small squamous suture ; (2) there is no optic foramen ; and 
(3) the presphenoid is an autogenous bony element. The posterior sphenoid has about 
the same extent, axially; but laterally, it is one third larger, or nearly. The basi¬ 
sphenoid (b.s.) is as long as the presphenoid ( p.s.), and is about one-fourth wider, on its 
upper face (fig. 4). It has a small open pituitary space (as an Erinaceus and Sorex), 
but the seat of the sella is shallow ; there are no tympanic alas. The alisplienoids (al.s.) 
are larger than they would seem to be, viewed merely from the upper surface (fig. 4); 
they are best seen from below (fig. 5). The 3rd branch of the 5th nerve (V 3 .) passes 
