202 ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 



large and complex brain well fills the flat oblong brain-cavity, in front of which we now 

 see the exquisite folds of the nasal sac (o/). 



Looking at the basis cranii (PI. XXXVI. fig. 6, and PI. XXXVII. fig. 1), we find 

 that the notochordal region is now a broad subquadrate tract of cartilage, the two moieties 

 of which have coalesced ; there is a very small cone of gelatinous tissue left. 



The investing mass {i t') behind is developed into two lobes for articulation with the 

 " atlas ; " and in front its extremities are rounded, the whole basilar palate being emar- 

 ginate in front. This emargination and the space between the preepituitary cartilages 

 {tr) make together a thin subpituitary space of cartilage of a lozenge-shape. The 

 " internal carotids " (?'. c) pierce this space at its broadest part. Opposite these pas- 

 sages the anterior pair of cartilages [tr) curve outwards into a short flat cornu. This 

 cornu is the " elbow " of the trabecular bar (fr) ; it is largely separated from the side of 

 the basis cranii, being bowed out : this is well seen in the Porbeagle (Lamna cornuhica), 

 and also in Carchanas glaucus (see " Huxley and Hawkins's Atlas," pi. 5. fig. 4). In the 

 basal figure of this skull of the embryo Dog-fish (PI. XXXVII. fig. 1, tr) the trabeculse 

 are seen to be of great size and remarkably flattened out ; they expand beneath the eye- 

 balls, and send out an antorbital process on each side. In front of these processes the 

 trabeculse are suddenly narrowed, and end between the nasal sacs (na) in a pair of short 

 horns (cornua trabeculse, c.tr). These are the distal extremities of the trabeculae, 

 which here have a basal or azygous piece, the prsenasal rostrum or " basitrabecula " 

 (b.tr). This is exactly like what has been described in the Bird (" Fowl's Sknll," pis. 81- 

 84, pn) ; its direction is a gentle curve upwards (Pi. XXXVII. fig. 4, b.tr). 



The trabeculae at their extremities are 4-winged in section; for the narrowed thick lower 

 part sends upwards, but more outwards, a thin broad lamina (PI. XXXVI. figs. 3 & 4 ; 

 PL XXXVII. fig. I, c.tr) ; this is the " trabecular crest," and is a very important struc- 

 ture. In vertebrates with high skulls and a well-developed meso-ethmoid (perpendicular 

 plate and nasal septum), the nasal sacs come close together, and their inner plates not 

 only coalesce with each other, but also with the ascending trabecular crests, to form the 

 single solid septal plate. 



Here the distance of the olfactory sacs from each other leads to the correlated diver- 

 gence of the trabecular crests, which coalesce with the inner walls of the nasal dome. 

 Each outspread crest gives off a small sigmoid cornu ; and these two horns curl inwards 

 towards the basitrabecular rostrum. To help in the interpretation of these parts the 

 nasal domes have been emptied of their olfactory folds, and the valvular " labials " 

 turned a little aside (PL XXXVI. fig. 4, and PL XXXVII. fig. 1, na, I, 1, 2, 3). 



The dome-shaped olfactory cartilages are not only joined by coalescence to the 

 trabecular crests ; they are also confluent with an important pair of cartilaginous bands, 

 namely the superorbital tracts of cartilage ; these have already been described in the 

 Salmon ("Salmon's Skull," pi. 4, s.ob), and are well shown by Dr. Traquair in the 

 Polypterus (Journ. of Anat. and Phys. voL v. pi. 6. figs. 2 & 3). They are early seen 



