SKULL AND VISCERAL SKELETON OF THE GREENLAND SHARK. 297 



the dorso-lateral connections were also extensive, and they largely encroached on the 

 dorsal and lateral spaces, reducing them to wide canals. 



In other Elasmobranchs, a membranous partition stretches across the space between 

 the cranial cavity and the pre-frontal fossa. 



The pre-frontal fossa (Pf). — This is an elongated deep fossa, which, as already 

 mentioned, is open above (Fig. 1), and communicates with the cranial cavity posteriorly. 

 It is deeper behind than in front, its floor slopes upwards to the rostrum, and towards 

 its upper part its margins curve inwards towards each other. 



The parietal fossa (P). — This fossa has already been noticed (Figs. 1 and 5). In a 

 longitudinal section of the skull, the opening of a vestibular aqueduct is seen posteriorly 

 at the side of the floor of the fossa. 



The Notochord (C). — In Lsemargus, as in some other sharks, the notochord is 

 persistent in the cranial floor. It passes forwards from the vertebral column to the 

 vicinity of the pituitary fossa, and approaching the dorsum sellse, curves rather abruptly 

 upwards. Its anterior extremity in some cases is directed forwards, in others it 

 curves backwards, but in all cases it terminates just below the perichondrium of the 

 cranial floor. The direction of the cranial notochord of Lsemargus bears a greater 

 resemblance to that of Hexanchus or Heptanchus, than to that of such forms as 

 Acanthias or Centrophorus calceus. 



The Visceral Skeleton (Plates I. and II.). 



The visceral skeleton consists (1) of the usual segmented hoops or arches, placed in 

 succession one behind the other, and (2) of cartilages standing in relation to these. Of 

 these arches there are seven. The first is the mandibular, the second the hyoid, and 

 the remaining five are the branchial arches. 



The Branchial Arches (PI. I. Figs. 1 and 2, PI. II. Figs. 2 and 3). — These arches 

 gradually decrease in size from before backwards. A typical arch, e.g. the third 

 (PI. II. Fig. 2), consists on each side of the four segments which are usually 

 found in Selachians. From above downwards, they are as follows, — (1) pharyngo- 

 branchial (P&3), (2) epi-branchial (E3), (3) cerato-branchial (Kr3), and (4) hypo- 

 branchial (H3). A series of cartilages, the basi-branchials (PL II. Fig. 3, B1-B8), 

 occupy a mid-ventral position between the lower ends of the lateral portions of the 

 arches. 



In several preparations of the visceral skeleton of Lsemargus which I have examined, 

 I found the typical number of segments in the first four arches, while in other preparations 

 I noticed that only the first, third, and fourth arches possess the typical number, the 

 hypo-branchials of the second arch having fused together to form a single transversely 

 placed plate of cartilage (PI. II. Fig. 3, H2). In some cases there is an intermediate 

 condition in which this cartilaginous plate is incompletely divided. 



In all my dissections I found that the fifth arch possessed only two segments on each 



