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, 
New South Wales (Lower Hawkesbury Formation), and North 
America. Plewracanthus, 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 
cartilage, 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. Diplacanthus), form a dorsally incomplete 
arch, apparently corresponding to a secondary pectoral girdle 
for the support of the stout pectoral spines, in which elements 
1 A. Fritsch, Fauna der Gaskohle in Bohmen, ii. Prague, 1889; Kner, SB. 
Akad. Wiss. Wien Math.-Naturw. Cl. lvii. Pt. i. 1868, p. 290; Traquair, Geol. 
Mag. (3), v. 1888, p. 511, and (4) i. 1894, p. 254. 
