OP THE SKULL IN SHAEKS AJSTD SKATES. 219 



these are marks of the primary pituitary space {py), the soft tract between the bowed 

 edges of the trabeculae. The trabeculse have developed a common basal piece, the 

 " basitrabecular " rostrum {b.tr) ; but this is not segmented off as a distinct bar. Where 

 the trabeculse have so completely coalesced in front, forming also their basal rostrum, 

 they turn upwards into the frontal wall of the face, and finish the cranial floor. The 

 nasal sacs are hollow inverted cups of cai'tilage, with their downturned mouth stopped 

 largely by " labials." The rim of the cup is strong on the outside, and also gives 

 attachment to an ethmo-palatine cartilage {a.o). 



The roof, also, of the nasal sac is modified in its form by reason of the engrafting upon 

 it of a large bowed cai'tilage, the " superorbital " (fig. 4, s.ob), a cartilage which has but 

 little independence of growth, but the substance of which early appeal's in the embryo. 

 The lower edge of this cartilage, mesiad of the eye-ball, is continuous with the trabe- 

 cular crest ; and the " tegmen cranii" grows directly from it towards the mid line of the 

 roof. The roof and side walls are analogous to the upper portion of a vertebral arch. 



Here, however, in the Skate, the tegmen is largely undeveloped; the brain-sac is 

 permanently membranous above in front, and rests upon the laminar trabeculee. Then, 

 just in fi'ont of the nasal region, there is a cartilaginous beam thrown over; but 

 it is narrow, and thence to the ear-sacs the roof is bare of cartilage. Behind, the 

 tegmen reappears, and helps the superoccipital to roof in the hinder brain between the 

 auditory masses [au). The superorbital arc grafts itself also on these sacs ; hence the 

 compound region, which ossifies separately in the " Teleostei," behind the orbit, the 

 so-called postfrontal or sphenotic. 



The clear but cheese-like cartilage shows the three canals through its walls {a.s.c, 

 k.s.c, p.s.c) ; and where the anterior and posterior of these unite, the " aquseductus vesti- 

 buli " is seen open. 



Behind, a mass of notochord is still to be seen, and the parachordal cartilages project 

 backwards outside, to form the occipital condyles {oc.c). The epiotic elevation over the 

 junction of the anterior and posterior canals is slight ; the pterotic ridge outside the 

 horizontal canal is well developed. 



The oral and phaiyngeal visceral arches (the first or trabecular has been described as 

 part of the chondrocranium) are nowhere more instructively developed than in the Rays. 



Keeping the eye upon the early condition of these parts (Pis. XXXV. and XXXIX.), 

 we shall see what metamorphic results have been brought about. 



The apex of the first postoral or mandibular arch has been developed as a distinct 

 crescentic cartilage, the " spiracular cartilage " or metapterygoid (PI. XL. fig. 4, mt.pjg) ; 

 it is attached below the sphenotic process, and behind the fifth nerve ; it is the bearer 

 of the " pseudo-branchia " {ps.br), lies in the anterior wall of the first cleft, and answers 

 to the " otic process" of an Amphibian. This detached suspensorium is joined to the 

 quadrate region by a ligament ; it answers to the hinder fork of the visceral rod. The 

 larger anterior fork, the posterior extremity of which is part of the main descending bar 



