CENTROSCYLLIUM NIGRUM. 29 



labial cartilages, Plate IV., fig. 1. The two on tlie upper jaw, the pre- 

 maxillary and the maxillary, are quite slender, and the anterior is shorter 

 tliiin the other. The one on the lower jaw is much stronger every way 

 and widens toward its anterior extremity. In tlie 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. Tlie hinder basibranchial is 

 a large broad plate that narrows backward to a point ; at each side it 

 directly supports the lower ends of the fourtli and the fifth ceratobranchials. 

 If this condition is compared with what obtains in Squalus acanthias, see 

 Gegenbaur, 1872, Das Kopfskelet der Selachier, PI. XVIII., 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 fiv^e 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 bearing one or two radlals, 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 radlals that are in most cases segmented near 

 the distal end ; anteriorly against the end of the pelvic element three or 

 four additional radlals have coalesced to form a single large plate of 

 cartilage. 



The viscera were destroyed. A few remnants are figured on Plate V., 

 fiifs. 3 and 6. Fiirure G shows the heart with three series of valves in the 



