SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 301 



dorsal and a ventral extra-branchial. The extra-branchials lie for the most part posterior 

 to the arches to which they belong. 



The gill rakers. — These, for the most part, are pointed or blunt processes of cartilage, 

 lying chiefly on the inner surfaces of the cerato-branchials. Their bases rest on, and in 

 some cases are continuous in this position with the cartilage of the arch to which they 

 belong. Their free extremities project into the buccal cavity. 



The Hyoid Arch (PI. I. Figs. 1 and 2, PI. II. Fig. 3). 



This is a massive arch, and consists of a mesial and two lateral portions. Each of 

 the lateral portions of the arch consists of the usual cartilaginous segments — an upper 

 or hyo-mandibular (Hm), and a lower or cerato-hyal (Kh). The hyo- mandibular cartilage 

 is short and broad, and when the parts are in apposition, stands out horizontally 

 from the skull. The inner extremity of the cartilage presents an oblique surface with 

 two heads (g,g f ) which are separated from each other by a shallow groove. The 

 heads, of which the posterior is the larger, articulate with the two surfaces which 

 exist in the cranio-hyoid depression of the skull (PI. I. Fig. 4, j,f). At the outer 

 extremity of the hyo-mandibular, a strong process, which may be called the suspensorial 

 process, projects forwards towards a similar process arising from the hinder part of the 

 lower jaw. The two processes are connected by a ligament, in which a cartilage 

 corresponding to an interarticular cartilage is imbedded. Some other sharks also possess 

 a cartilage in this position. There is a depression on the lower surface of the suspensorial 

 process, for articulating with the upper extremity of the cerato-hyal. 



Cerato-hyal (Kh). — This is an elongated curved bar of cartilage, and has a direction 

 from without downwards, forwards, and inwards. A rounded prominence, which articulates 

 with the depression on the under surface of the suspensorial process of the hyo-mandib- 

 ular, rises from its upper extremity. The lower extremity of the cerato-hyal passes below 

 the outer part of the basi-hyal (Bh), and rests in a depression which is there situated. 



Basi-hyal (Bh). — This is a block-like cartilage which lies between the lower ends of the 

 cerato-hyals. Its anterior border is convex and its posterior is concave ; its dorsal surface 

 is flattened, while its ventral presents a concavity. Postero-laterally, it is produced into 

 two cornua, to which the lower extremities of the cerato-branchial cartilages of the first 

 gill arch are bound by connective tissue. The first pair of hypo-branchials (Hi) lie at 

 the hinder part of the cerato-basi-hyal joints, and they are also bound by connective 

 tissue to the cartilages forming these joints. 



Hypo-hyal (Hh). — A nodule of cartilage is situated at the fore and upper part of 

 each cerato-basi-hyal joint.* The cartilage occupies a position similar to that which the 

 hypo-branchial (Hi) of the first gill arch does in front of the first cerato-branchial 

 cartilage. It does not appear that these nodules have been noticed before in any other 

 shark. They are evidently hypo-hyal cartilages. 



* These cartilages were absent in two of the sharks. 



