440 FISHES CHAP. 



Fam. 1. Pleuracanthidae. — The single family included in 

 the group ranges from the Lower Carboniferous to the Lower 

 Permian. Within these limits the family is widely distributed 

 in different formations in Great Britain, Continental Europe, 

 ISTew South "Wales (Lower Hawkesbury Formation), and North 

 America. PI eur acanthus, of which complete skeletons and skulls 

 have been found, is the best known genus. 



Order III. Acanthodei. 



The Fishes comprising the Acanthodei ^ may be regarded as a 

 highly specialised and terminal offshoot from some primitive race 

 of early Elasmobranchs. The Elasmobranch kinship of the Acan- 

 thodei is indicated by their exoskeleton of shagreen tubercles ; 

 the completely heterocercal tail ; the absence of an operculum, 

 the external gill-clefts apparently being exposed ; the position 

 of the lateral line of the trunk between two rows of shagreen 

 denticles ; the nature of the powerful spines in connexion with 

 the dorsal and anal, and the pectoral and pelvic fins ; and the 

 formation of the hard parts of the skeleton, not by ossification 

 involving the presence of bone-cells, but by the calcification of 

 cartilag^.or of more superficial membranous or fibrous tracts. 

 On the other hand, it may be noted that the Acanthodei appear 

 to have undergone much specialisation on lines in some respects 

 parallel to those which have marked the evolution of the 

 Teleostomi, but by methods which are simply an exaggeration of 

 features normally characteristic of Elasmobranchs. Perhaps the 

 most striking illustration of this is to be seen in the develop- 

 ment of a species of secondary skull by an extension of a process 

 of calcification as distinguished from ossification. Hence the 

 presence of membrane-calcifications in relation with the upper 

 and lower jaws, whose development is proportional to the size of 

 the teeth they support, and of smaller investing plates of the 

 cranial roof Similar exoskeletal calcifications, when most com- 

 pletely developed (e.g. Bijdacanthus), form a dorsally incomplete 

 arch, apparently corresponding to a secondary pectoral girdle 

 for the support of the stout pectoral spines, in which elements 



^ A. Iritsch, Fauna dcr GaskoMe in Bolimen, ii. Prague, 1889 ; Kner, SB. 

 Ahad. Wiss. Wien Math.-Nalurw. CI. Ivii. Pt. i. 1SC8, p. 290 ; Traquair, Geol. 

 Mag. (3), v. 1888, p. 511, and (4) i. 1894, p. 254. 



