CENTROSCYLLIUM NIGRUM. 29 
labial cartilages, Plate IV., fig. 1. The two on the upper jaw, the pre- 
maxillary and the maxillary, are quite slender, and the anterior is shorter 
than the other. The one on the lower jaw is much stronger every way 
and widens toward its anterior extremity. In the branchihyal frame- 
work, Plate V., fig. 2, reduction has proceeded about as far as in any of 
the species mentioned above. The foremost hypobranchials have appar- 
ently consolidated with the anterior ceratobranchials. Behind the basihyal 
there are two basibranchials, the anterior one of which is short and joins 
the middle of the anterior border of the other one, separating the hypo- 
branchials of the third pair, and itself meeting the hinder extremities of 
those of the second pair on its forward end. The hinder basibranchial is 
a large broad plate that narrows backward to a point; at each side it 
directly supports the lower ends of the fourth and the fifth ceratobranchials. 
If this condition is compared with what obtains in Sgualus acanthias, see 
Gegenbaur, 1872, Das Kopfskelet der Selachier, Pl. XVIIL., fig. 3, it is 
found to be the case that in that species the branchihyals are rather less 
reduced, since it possesses three distinct pairs of hypobranchials, instead of 
only two, has its basibranchials separated by the hypobranchials instead 
of in contact, and has only the posterior pair of ceratobranchials, instead of 
the posterior two pairs, abutted directly against the sides of the hinder 
basibranchial. Thus in respect to the branchial skeleton the present species 
is the more specialized. On each side there are five extra-branchial car- 
tilages. The small subquadrangular spiracular cartilage is nearly divided 
into three short bars. 
The skeletal elements of the pectorals vary to some extent in individ- 
uals, see Plate V., figs. 2 and 4. Generally the propterygium, the meso- 
pterygium, and the metapterygium are comparatively large and about equal 
in size, the first bearmg one or two radials, the second three or four, and 
the third about ten, of which three or four of the posterior are unseg- 
mented. There is an elongate basal cartilage in the skeleton of the ven- 
tral supporting about fifteen radials that are in most cases segmented near 
the distal end; anteriorly against the end of the pelvic element three or 
four additional radials have coalesced to form a single large plate of 
cartilage. 
The viscera were destroyed. A few remnants are figured on Plate V., 
figs. 3 and 6. Figure 6 shows the heart with three series of valves in the 
