534 FISHES CHAP. 
which in Bothriolepis canadensis have their oral margins 
fringed by small “denticles”; it is possible that these plates 
represent the components of a secondary upper jaw. The dorsal 
armature of the trunk is shown in Fig. 522. Ventrally it is 
completed by a pair of anterior ventro-lateral plates and a pair 
of posterior ventro-lateral plates with a small median plate 
between the two pairs. Articulating with the anterior ventro- 
lateral plates by means of a complex hinge joint there is a pair 
of pectoral appendages of a kind entirely without parallel in 
any other vertebrated animals. Each appendage is completely 
encased by numerous suturally connected plates, and about the 
middle of its length there is a second movable joint. The 
appendages are hollow, and their cavities probably contained the 
muscles by which the limbs were moved, and the blood-vessels 
and nerves for their nutrition and innervation. A lateral line 
system of the normal type is present in P/erichthys, consisting of 
a lateral groove along the side of the trunk, and of supra-orbital 
and infra-orbital grooves, and post-temporal and infra-orbital 
commissures, on the head. The free portion of the body and the 
tail are invested by imbricated and finely tuberculated scales, which 
form fulcra in front of and behind the small dorsal fin. There 
are no pelvic fins. The caudal fin is heterocercal. 
Fam. 1. Asterolepidae.—The best known genera are Pterich- 
thys from the Lower Old Red Sandstone of Scotland and the 
Devonian of Eifel, and Bothriolepis, a more widely distributed 
genus which oceurs in the Upper Old Red of Scotland and 
Shropshire, and in the Upper Devonian of Russia and Canada. 
Two other genera, Asterolepis and Microbrachius, are also found 
in the Old Red Sandstone of Scotland." 
Beyond an uncertain and shadowy relationship to the Ostraco- 
dermi, and perhaps some points of resemblance to the Arthrodira, 
the Antiarchi stand alone among Craniates. Nothing is known 
of their origin; no intermediate forms link them to any other 
groups, and the high specialisation they have attained is sufficient 
to negative any idea that they can “be credited with any share 
in the evolution of the Fishes of more recent periods.” 
1 Traquair, Proc. Roy. Phys. Soc. Edinb. xi. 1891-92, p. 283. 
