222 5IE. W. K. PARKER ON THE STRUCTIJRE AND DEVELOPMENT 



crests close together, ready for fusion at the mid line : the distinct antorbital {a.o) is cut 

 through. In an antorUtal section (PL XLII. fig. 1), the nasal dome is seen to be 

 thickened by the superaddition of the engrafted superorbital arc; and, in this its hinder 

 part, the nasal sac is some distance from the trabecula; and the cranial cavity. The 

 trabecular crest has also received increment from the same " brow " of cartilage {s.oi) ; 

 and thus the deck is partly covered in — entirely for a short distance, a little further back. 

 Beneath, the pterygoid undergirders are seen, where they join by a fibrous band. 



A section (PI. XLII. fig. 2) behind the mouth [m) and through the eye-baU {e) shows 

 the squared form of this little barge, also that the brain at the time of hatching well 

 fills its cavity. The upper third of the " orbito-sphenoidal " side wall is due to the 

 thick brow-cartilage {s.oh), the rest to the trabecular crest ; the floor, thicker at this 

 part, is a trabecular commissure. 



The quadrate part of the upper jaw is here cut through, and also the whole extent of 

 the articulo-Meckelian bar, with its distinct symphysis, its projecting angle, and its 

 cupped articular surface for the rounded quadrate condyle. The mouth (m) is shown 

 as in a nearly closed state ; it has much mobiUty, because of its free double suspen- 

 sorium, its well wrought articulo-quadrate hinge, and its anterior and posterior fibrous 

 symphyses at the ventral extremities of the two arches. 



The last section (fig. 3) to be described is through the auditory sacs, somewhat 

 obliquely, near their posterior wall ; for it can be seen that the occipital arch is here 

 complete, crowned with a veritable spinous process, having neurapophysial sides that 

 form a gothic arch, and a broad base, on which lies the diminished and fading cephalic 

 notochord. 



On the right side the horizontal canal [h.sc) is cut through where it overarches the 

 condyle for the hyomandibular {hm). On the other side, the posterior canal is cut 

 through, the razor cutting further back ; and the " interhyal " ligament is seen carrying 

 the epihyal {e.htj) with its three branchial rays, below which part of the ceratohyal {c.hy) 

 is seen, with three more rays. On the right side, under the hyomandibular, most of the 

 ceratohyal is seen {c.hy) with its flat rays, and below it the little hypohyal {h.hy) 

 attached to the long slender hypobranchial belt of the first proper branchial arch. 



This figure shows the breadth of the pharynx whilst in a state of rest. 



Fourth Stage: Skull of adult Thornhack Skate (Raia clavata). 



The skull of the adult Skate may be said to be fiddle-shaped (Pis. XLI. and XLII.) ; 

 for it has very pinched sides, is very flat, and has a long stem or handle. The narrowing 

 of its body is to make room for the eye-balls ; the bulging parts are the auditory sacs 

 behind, and the wider nasal sacs in front ; and the projecting shaft is the enormously 

 developed basitrabecular rostrum, which in this and related types acquires its uttermost 

 development. 



The interauditory part only of the base of this choudrocranium is formed by the 

 " parachordal " cartilages'; from the postpituitary wall to the end of the snout all the 



