SUB-CLASS I 



SELACHII 



35 



Cretaceous (C loagidens, Pillet) and one existing species (C. mndeleti, M. and 

 H.) ; the others Tertiary. C. megalodon, Ag. (Fig. 70), and allied species are 

 represented by very large teeth in the Tertiary and later formations of 

 nearly all parts of the world, also on the bed of the existing oceans. " 



Cetorhinus, Blv. (Selache, Cuv.). Teeth very small and conical, without 

 lateral denticles. Claspers of male with horn-shaped spines, which have 

 been found in the Pliocene of England and Belgium. The existing " basking 

 shark." 



Sub-Order D. TECTOSPONDYLI. Hasse. 



Vertebral centra when comyletely formed ainphicoelous, strengthened hy secondary 

 plates of calcified tissue which are concentric with the constriäed primitive cylinder. 

 Five gill clefts on each side. Anal fin absent. Body usually depressed, with very 

 large pectoral fins forming a " disk." 



To the Tectospondyli belong the still existing rays, monk-fishes, and saw- 

 fishes, perhaps also the Palaeozoic families of Cochliodontidae, Psammo- 

 dontidae, and Petalodontidae. 



Family 1. Cochliodontidae. Owen. 



Palaeozoic fishes with few convex teeth, which are more or less arched, hollowed at 

 the base, punctate on the crown, and offen marJced with obtuse transverse folds or 

 grooves. A large posterior dental plate, above and below on each side, a smaller 

 (Uiterior dental plate, and one or more series of separate smaller anterior teeth. 

 llead and perhaps also the pectoral fins armed with paired tuberculated spines. 

 Carboniferous and Permian. 



Our knowledge of this most remarkable Upper Palaeozoic family is still 

 very incomplete. Of two genera (Helodus and Menaspis) the Impression of the 

 trunk is known ; of all the others only teeth or fin spines and, very rarely, 

 complete jaws occur. According to Jaekel we may place here the un- 

 symmetrical Ichthyodorulites, with tubercles, bosses, or spines and destitute of 

 a root, which have been described under the names of Oracanthus, Pnigeacanthus, 

 öladacanthus, Platyacanthus, Gamps- 

 acanthus, Physonemus, Stichacanthus, 

 etc. Owen has pointed out that the 

 deiitition in the typical genera 

 (Jochliodus and Streblodus corresponds 

 with that of a Cestraciont, but differs 

 from the latter in the fusion of two 

 or more transverse series of teeth into 

 continuous plates. Smith Wood ward 

 has thus proposed to place the typical 

 Cochliodontidae near the Cestraci- 

 ontidae, removing Menaspis *to the 

 Chimaeroid family of Squaloraiidae. 



Cochliodm, Ag. (Fig. 71). Two 

 strongly arched dental plates on each 



ramus of the mandible ; the posterior one very large, with a broad median 

 area bounded in front and behind by an oblique transverse furrow; the 

 antei-ior plate comparatively small, rhomboidal in shape. Symphysial series 



Fk;. 71. 



Cochliodas contortus, Aj?. Dentition, -Vi .nat. size. 

 Carboniferous Limestone ; Arniagh. A, Large i)0.sterior 

 tooth. B, Middle tooth ; anterior teeth wanting. 



