534 FISHES CHAP. 



which in Bothriolepis canadensis have tlieir 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. 322. 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 PtericMliys, 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 Bed Sandstone of Scotland and the 

 Devonian of Eifel, and Bothriolepis, a more widely distributed 

 genus which occurs in the Upper Old Eed of Scotland and 

 Shropshire, and in the Upper Devonian of Eussia and Canada. 

 Two other genera, Asterolepis and Microirachius, are also found 

 in the Old Bed 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." 



' Traquair, Proc. Roy. Phys. Soc. Minb. xi. 1891-92, p. 283. 



