SUB-CLASS I 



SELACHII 



23 



arches of mrtehral axis long and slender. Pedoral fins with long, segmented axis 

 from which smaller cartilages diverge on either side ; pelvic jins of male with appended 

 elasper ; caudal fin diphycercal. 



Family 1. Pleuracanthidae. Cope. 



Bodt/ slender, but slightly depressed .; mouth terminal. Teeth with two principal 

 divergent ciisps. Branchial arches provided with Clusters of minute denticle's. Dorsal 

 fin elongate, low, continuous along the hack from a point shortly hehind 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. Diplodus, Ochlodus, Aganodus, 

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



FiG, 40. 

 Restoration of Pleuracanthus gaudn/i, Brongn. Goal Measures ; Comineutry, Allier (after C. Brongniart). 



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

 branchial arches, the name Stemmatias (Stemmatodvs). 



Pleuracanthus, Ag. (Xenacanthus, Beyrich; "i Didymodus, 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 triangulär cartilages ; basi- 



11. 1889, p. l2Z.— Fritsch, A., Fauna der Gaskohle, vol. II. Prague, \^^9>-^^.—Goldfuss, A., Neues 

 Jahrb. 1847, p. 404, and Beiträge zur vorweltlichen Fauna des Steinkohlengebirges, p. 23, pl. v. 

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

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



