CLASS I 



PISCES 



L5 



Pig. 26. 



Pectoral tin of Ceratodus forsteri, Krefft. Recenl ; 

 Queensland, a, Basal cartilage; b, Fin; C, Pectoral 



arch. 



type most closely approaching the primitive fin (archipterygiuni), 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 



which cartilaginous rays diverge on 



each side. The extinct Selachian genus 



Pleur acanthus also possessed a central 



axis in the pectoral fin ; but in all the 



other Selachii the cartilaginous rays 



are in one series. In the latter there 



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 always 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. Pohjpterus (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 

 Teleostei (Fig. 28, C) consists almost invariably of four to 

 five similar, flattened, bony pieces, with which a varying 

 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- 



scl 



Fia. -J7. 



Pectoral tin of Squa- 

 tina vulgaris, Risso. 

 With outer dermal rays 

 removed, ms, Meso- 

 pt irygium ; rat, Meta- 

 pterygium ; p, Pro- 

 pterygium ; /', Radials. 



Fig. 28. 



Pectoral arch and tin of A, Heptanchus (Selachii). /;, Polypterus (Ganoidei). C, Salmo (Teleostei). d, clavicle ; 

 co, Coracoid ; fs, Outer dermal tin rays (exoskeletal) ; ms, Mesopterygium ; ml, Metapterygium ; p, Propterygium ; 

 pel, Postclavicle ; pt, Post-temporaf ; /■, Basal rays (endoskeletal) ; sc, Scapula : scl, Supraclavicle. 



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 



