SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 299 



hyoid arch, to which it is bound by connective tissue. The lower extremities of the 

 cerato-branchials of the second, third, and fourth arches are connected with hypo- 

 branchials (H2, H3, H4) respectively, while those of the fifth pair (Kr5), which differ 

 considerably from those of the other arches, are bound, one on either side, to a large basi- 

 branchial plate (B5). 



Processes project forwards from the lower ends of all the cerato-branchials, but those 

 of the first arch are very feebly developed. Each process on the second, third, fourth, 

 and fifth arches overlaps the portion of the cerato-branchial lying in front of it. Fossae, 

 for muscular attachments, similar to those on the inner faces of the epi-branchials, are 

 also found in corresponding positions on the first four cerato-branchials, close to their 

 upper extremities. 



Hypo-hranchials (PI. II. Figs. 2 and 3, H1-H4). — There are four pairs of these 

 cartilages, and they stand in relation to the first four branchial arches. The first pair 

 (Hi) are very small cartilages, lying, as is already mentioned, at the fore part of the 

 lower extremities of the cerato-branchials of the first gill arch, to which, in the recent 

 state, they are bound on the one hand, and on the other to the hyoid arch, by connective 

 tissue. In some sharks, e.g. Hexanchus, these pair of hypo-branchials are absent. The 

 second pair of hypo-branchials (H2) may be fused together, as in Scymnus, and form a 

 transversely placed plate of cartilage which lies between the lower ends of the cerato- 

 branchials of the second gill arch. A process projects backwards from this plate, and in 

 some cases a small process projects forwards from the middle of its anterior border. In 

 other cases the anterior border of this plate presents a straight edge, as is shown in Plate 

 II. Fig. 3. In several preparations I found this plate consisting of two symmetrical pieces. 



The third and fourth pairs of hypo-branchials (H3, H4), which have a slightly back- 

 ward direction, much resemble each other. Their outer extremities are connected with 

 cerato-branchials and their inner with basi-branchial cartilages. 



Basi-branchial cartilages (PI. I. Fig. 2, PI. II. Figs. 2 and 3, Bl-8). — These are a series 

 of cartilages differing from each other in shape and size, varying slightly in number, and 

 forming a broken line in the mid-ventral position. I found in most of the specimens of 

 Lsemargus which I examined a larger number of these cartilages than have been described 

 in any other Elasmobranch. Heptanchus has five of these, but Lsemargus has a number 

 ranging from six to eight. The first basi-branchial (Bl), when present, is a small nodule 

 of cartilage which lies in the interval between the basi-hyal cartilage (Bh) and the hypo- 

 branchial plate (H2) of the first gill arch. Cestracion, so far as I am aware, is the only 

 other Elasmobranch yet described which has a basi-branchial in this position. In two 

 cases in which this cartilage was absent in Lsemargus, I noticed that the hypo-branchial 

 plate (H2) of the first gill arch had a process projecting forwards from the centre of its 

 anterior border, This is interesting, as there may be a possibility that in these cases, 

 the first basi-branchial is fused with the hypo-branchial plate. In some cases, however, 

 I found that the first basi-branchial was altogether absent, and the hypo-branchial plate 

 presented a straight anterior edge. The second basi-branchial cartilage (B2) is placed 



