xvii ELASMOBRANCHII PLAGIOSTOMI 443 



in one species {Xot'uhmus cinereus), which exhibits a feeble 

 asterospondylism in the caudal vertebrae. Skull amphistylic. 

 Teeth unlike in the two jaws ; those in the upper jaw usually 

 with a large central cusp and smaller lateral cusps ; those in the 

 lower jaw comb-like, each consisting of numerous graduated 

 pointed cusps inclining in the same direction, and supported on 

 a long basal plate. 



The very few species included in this family are widely 

 distributed in the tropical and subtropical regions of the Atlantic 

 and Pacific Oceans. N'otidanus (Heptanchus) cinereus, which has 

 seven branchial arches and clefts, inhabits the Mediterranean 

 and Atlantic. JV. {Hexanchus) griseus, with six branchial arches 

 and clefts, has a similar distribution, but besides being an 

 occasional visitant to the British coasts, it is .not uncommon at 

 Cuba in the West Indies. It is said to grow to a length of 

 26 feet. 



Fossil remains of JVotidamis, principally teeth, occur in the 

 Middle and Upper Jurassic, in the Cretaceous, and in the Eocene 

 and Pliocene of England and the Continent. 



Fam. 2. Chlamydoselachidae (Frilled Sharks). — Body much 

 elongate. Median fins as in Notidanus. Mouth nearly terminal. 

 Nostrils lateral, nearly terminal, and without oro-nasal grooves. 

 Branchial arches and clefts six. The outer margins of the inter- 

 branchial septa are produced into overlapping cutaneous frills, 

 the first of which is developed from the hyoid arch and overlaps 

 the hyobranchial cleft, like a rudimentary operculum. Vertebral 

 column as in the preceding family, but in the hinder part of the 

 trunk the notochord is unconstricted and uniform in diameter, 

 centra being absent. Skull hyostylic. Lateral line an open 

 groove. Teeth alike in both jaws, each consisting of a broad 

 basal plate supporting three slender curved cusps, separated by a 

 pair of much smaller cusps. 



The only living species known is Clilamydoselachus anguineus 

 (Fig. 252),^ which occurs in the Pacific near Japan, in deep water 

 off Madeira, and also off the Azores and the coast of Norway. 

 It reaches a length of 4 to 5 feet. Teeth from the Pliocene 

 deposits of Tuscany have been referred to an extinct species, 

 C. lawleyi. 



' Garman, Bull. Mus. Comp. Zool. Harvard, xii. No. 1, 1885, p. 1 ; Giinther, 

 Chall. Hep. Zool. xxii. 1887, p. 2. 



