740 MR. C. TATE REGAN ON [Juiie 19, 



features, and form an isolated group without very close relation- 

 ships to other Euselachii. 



The gill-clefts retain their primitive position, as in the Lam- 

 nidte and Squalidee, the last being in front of the base of 

 the pectorals. It is probable that their large number is a 

 primitive feature, but the recent discovery of a P7'istio2ohor%is with 

 six gill-clefts shows that the importance of this character has 

 been overestimated. In this case, however, it is accompanied by 

 a very generalised arrangement of the branchial skeleton. 



The snout is supported by a simple anterior prolongation of the 

 cranium. 



In Ghlamydoselachus the notochord is unconstricted, except 

 anteriorly, where there are a few calcified rings. In HexancJius 

 the notochord is constricted by annular thickenings of the carti- 

 laginous sheath, there being no calcification. In HeptrancMas 

 the notochord is constricted vertebrally by a series of calcified 

 rings, which assume more and more the form of a double cone 

 towards the tail. In the caudal region, secondary calcification 

 may give rise to four, six, or eight short rays, radiating from the 

 central double cones. 



The dorsal and anal fins have numerous pterygiophores, which 

 in Gldamydoselachtis are rather irregularly arranged and exhibit 

 some fusion, especially basally. In Heptranchias the radials are 

 regular and the basals of each fin are fused to form, two large 

 cartilages (text-fig. 120, B, p. 747). 



The pectoral fin has a rather peculiar and quite unique structure.. 

 The propterygium is rather small, but broad, and distally forms a 

 convex surface for the articulation of the mesopterygium, which 

 it partly excludes from the pectoral arch. The mesopterygium is 

 well-developed, and extends to the anterior edge of the fin distally 

 to the propterygium. In Chlamydoselachus it is smaller than in 

 Hexanclius and Heptranchias, and bears fewer radials than the 

 metapterygium, which is composed of a long proximal and a short 

 distal segment. In the Hexanchidfe the metapterygium consists of 

 a very short proximal and a long distal segment ; it bears about as 

 many radials as the mesopterygium. The radials are simple and 

 composed of from three to five segments (text-fig. 119, D, p. 743). 



The mixopterygia are of the Squaloid type, the axial cartilage 

 being cylindrical and pointed, the ventral marginal cartilage 

 short and distal in position. It is evident that this is a more 

 primitive structure than that of the Galeoid Sharks. 



The Hexanchoidei resemble the Galeoidei in the absence of fin- 

 spines. They resemble the Squaloidei in the structure of the 

 cranium and of the mixopterygia. These features, which they 

 have in common with the Sharks of the two other suborders, are 

 undoubtedly primitive and indicate only the derivation of all three 

 from the same stock. Their peculiar characters may be either 

 primitive, as in the case of the vertebral column and the gill- 

 clefts, or specialised, i. e. the absence of the first dorsal fin and 

 the structvire of the pectoral. 



