CLASS I 



PISCES 



15 



In the latter there Queensland, a, Basal cartilage ; 6, Fin ; C, Pectoral 

 arch. 



type most closely approaching the primitive fin (archipterygium), we observe, 

 fixed to the mainly cartilaginous pectoral girdle, two large pieces of cartilage, 

 followed by a long row of smaller 



cylindrical or square segments, from ^^^ "'^^ ^^^^^^^^^^'^'r-^,,. ^ P\ ■ 



which cartilaginous rays diverge on '^^££^ ''^^^^^"- " V j 



each side. The extinct Selachian genus "^^^^^^S^ ^ ' '"^ ] 



PI eur acanthus also possessed a central ^^^^^ ' ^ '^~^-^-^, '^ 



axis in the pectoral fin ; but in all the p^^ ,g 



other Selachll the cartilaginous rays Tectovnl &n of Ceratoclus forsteri, KieSt. Recent; 



are in one series. 



are three large adjacent basal cartilages, 

 the promeso-, and metapterygium, connected with the pectoral girdle, and 

 each bears several rays consisting of pieces of cartilage (Fig. 27). The 

 metapterygium is alwaj^s the stoutest ; the two others are 

 often much reduced or may even be completely absent. The 

 dermal part of the fin is itself always supported by numerous 

 horny filaments in pairs. 

 ^^ , In the Ganoidei, and still more in the Teleostei, the 



^^^/ / segments radiating from the basal pieces are extremely 

 ^' " degenerate. Polypterus (Fig. 28, B) retains the three basal 



pieces of the Selachii, but in most of the other Ganoids the 

 metapterygium forms almost the sole support for the fin — 

 the meso- and propterygium are atrophied. Two or three 

 rays, however, enter the same row as the basal pieces and 

 are connected with the shoulder girdle. 



In the same manner the base of the pectoral fin in the 

 With outer dermal rays Teleostei (Fig. 28, C) consists almost invariably of four to 



removed, flw, Meso- ^ • •^ n . , ^ ^ • ^,^ ^ • ^ 



pterygium; mt, Meta- nve Similar, flattened, bony pieces, with which a varying 

 pterygium "r'Rad^ais number of short little cartilages are connected. In propor- 

 tion as the peripheral parts of the primary fin skeleton 

 degenerate, ossified fin rays are developed as secondary structures on each 

 side of the dermal fin expansion. 



The so-called pelvic girdle of the hinder extremity is considerably simpler 

 than the pectoral girdle. Among the Selachii it appears as a paired or un- 



PectoraL fln of Squa 

 tina vulgaris, Eisso, 



Pectoral arch and tin of ^, Hcptanchus (Selachii). B, Polypterus (Ganoidei). C, Salmo (Teleostei). d, Clavicle ; 

 eo, Coracoid ; fs, Outer dermal fln rays (exoskeletal) ; ins, Mesopterygium ; mt, Metapterygium ; p, Propterygium ; 

 ■pd, Postclavicle ; pt, Post-temporal ; r, Basal rays (endoskeletal) ; sc. Scapula ; sd, Sujjraclavicle. 



paired rod pierced by nerve foramina, either lying in the longitudinal axis of 

 the body or forming an arch, convex in front, concave behind. To this girdle 

 are attached the two basal pieces (pro- and metapterygium) of the pelvic fin, 

 which bear a single series of cartilaginous rays on their outer side. Among 



