SUB-CLASS I 



SELACHII 



23 



arches of vertebral axis long and slender. Pectoral fins with long, segmented axis 

 from ivhich smaller cartilages diverge on either side ; pelvic fins of male with appended 

 clasper ; caudal fin diphy cereal. 



Family L Pleuracanthidae. CojDe. 



Body slender, but slightly depressed ; month terminal. Teeth with tivo principal 

 divergent cusps. Branchial arches provided with clusters of minute denticles. Dorsal 

 fin elongate, low, continuous along the hack from a point shortly behind the head ; 

 slender fin supports more numerous than the neural spines. Lower Carboniferous, 

 probably to Upper Triassic. 



The Pleuracanthidae occur in the coal-bearing strata of the Carboniferous 

 system and in the Lower Permian of Europe and North America. One 

 species is also known by a nearly complete skeleton from the Hawkesbury 

 Formation of New South Wales. For the isolated teeth of Pleuracanthus 

 various generic names have been proposed, e.g. Diplodns, Ochlodus, Aganodus, 

 Pternodus, Triodus, Thrinacoclus ; for isolated spines, the generic names Compsa- 



;Fia. 40. 

 Restoration of Pkuracantlnis gaudryi, Brongii. Coal Measures ; Commentry, Allier (afterX'. Brongniart). 



canthus, Orthacanthus, and Lophacanthus ; for the clustered denticles of the 

 branchial arches, the name Stemmatias (Stemmatodus). 



Pleuracanthus, Ag. {Xenacanthus, Beyrich; 1 Bidyviodus, Cope), (Figs. 40-43). 

 Head broad, semicircular in front, and provided behind with a long, slender 

 dermal spine, which is hollow, either rounded in section or antero-posteriorly 

 compressed, and bears a double longitudinal series of recurved denticles (Fig. 

 42). Teeth (Fig. 43) with two large divergent cusps fixed on a stout common 

 base, and frequently a small third cusp between these. Five branchial clefts 

 laterally placed in advance of the stout and broad, arched pectoral girdle, of 

 which the two halves are fused ventrally but separated dorsally. Pectoral 

 fins large, articulated with the postero-inferior angle of the pectoral girdle, 

 and consisting of a segmented axis fringed on either side by cartilaginous rays. 

 Pelvic supports in the form of a pair of separate triangular cartilages ; basi- 



II. 1889, p. 123.— Fritsch, A., Fauna der Gaskohle, vol. II. Prague, 1888-89.— Goldfuss, A., Neues 

 Jahrb. 1847, p. 404, and Beitrage zur vorweltliclien Fauna des Steinkohlengebirges, p. 23, pi. v. 

 Figs. 9, 10. — Kner, R., Sitzungsb. k. Akad. Wiss. Wien, math.-naturw. CI. vol. 551, 1867, p. 540, 

 with 10 plates. — Koken, E., Sitzungsb. Ge§. naturf. Freunde, Berlin, 1889, p. 77. 



