OP THE SKULL IN SHABKS AND SKATES. 229 



In Birds an " os uncinatum " also often occurs, which is evidently the true homo- 

 logue of the antorbital or eth mo-palatine of the Skate and of the Amphibians. Birds 

 also possess " labials." 



But, to return to the Ichthyopsida, I may mention that the Batrachia, which, like 

 the Selachians, appeared most evidently to show that there was no proper distinct 

 palatine arch, can now be cited as yielding what appear to me to be convincing proofs 

 of the existence of such an arch, whose suspensorial point is the ethmoidal projection 

 of the trabecula. 



At first the suspensorium of a Batrachian is quite distinct from the trabecular bar ; 

 but soon after hatching it becomes attached both before and behind — before to the 

 trabecular elbow, and behind to the ethmoidal region of the bar (see " Frog's Skull," 

 pi. 5. figs. l-4:,pff). 



If, in this condition, segmentation had taken place across this short conjugational 

 bar, the upper half would have belonged to the trabecula, and the lower half to the 

 suspensorium. 



Instead of this, in the Frog, it grows into a long arch, which projects forward beyond 

 its ethmoidal part, and is continuous with the suspensorium behind. 



In the Toad, however, (see " Batrachian Skull," part 2, pi. 54. figs. 3, 4), it is seg- 

 mented ofi", not only from its ethmo-trabecular attachment, but also from the pterygoid 

 process of the suspensorium, and becomes what its counterpart is at first in the Urodeles 

 and Skates, namely a distinct ethmo-palatine visceral piece. 



C. Comparison of the Selachian Skull with that of Ganoids and Teleosteans. 



In the lower kind of Ganoids, such as the Sturgeon (see Month. Micro. Journ., June 1, 

 1873, pi. 20, pp. 254-257), the skull becomes hyostylic in the highest degree, and the 

 hyomandibular has its lower third segmented ofi^, and separately ossified as a large 

 symplectic segment. 



The pterygo-quadrate cartilages are not unlike those of the Skate, but more arched, 

 ultimately having three bony tracts in them, namely the pterygoid, palatine, and meso- 

 pterygoid. 



But there is only a single counterpart of the two spiracular cartilages. It is an 

 arched or convexo-concave plate, lying over the supero-posterior part of the tubular 

 mouth; it is rounded and thick behind, and thin and angular in front, where it fits 

 in beween the right and left pterygo-quadrate bars ; it is a double " pedicle." 



In Polypteriis (Traquair, Journ. of Anat. and Phys. vol. v. plate 6), the pterygo- 

 quadrate is a large bar of cartilage more or less ossified by a metapterygoid, quadrate, 

 pterygoid, and mesopterygoid centre. But the top of the metapterygoid is very low 

 down ; and the hyomandibular has a small postero-superior osseous centre, and a large 

 bony tract which takes in all the rest of the bar, without an inferior " symplectic " 

 bony centre. 



VOL. X. — PART IV. No. 6. — March \st, 1878. 2 k 



