348 A HISTORY OF FISHES 



the head and trunk enclosed in bony plates, those on the hinder 

 part of the body being small and having the general appearance 

 of scales. The best-known genus is Pteraspis, in which the form 

 of the mouth has been recently made out. This is a transverse 

 slit on the lower surface of the head, surrounded by bony plates 

 bearing minute denticles, but with no trace of any structures 

 that might be interpreted as true jaws. Unlike the other two 

 sub-classes, the gills open to the exterior by a single aperture 

 on either side, as in some of the existing Hag-fishes (Mjyxine), 

 and the nostrils are paired — a primitive feature. Several curious 

 types of Heterostraci have been described, some of them box-like, 

 with broad rounded snouts and short thick tails (Fig. 123B), 

 others with the head-shield prolonged to form a pointed 

 rostrum. Their generally flattened form and rigid head and 

 trunk are evidence of sluggish habits, and they probably lived, 

 like the Cephalaspids, at or close to the bottom. 



Before leaving the Marsipobranchs mention may be made of a 

 curious little organism found in large numbers in the Caithness 

 Flagstones (Lower Devonian) near Thurso. This is generally 

 an inch or two in length, and has been named Palaeospondylus 

 gunni. The fossils are well preserved, and this creature can be 

 seen to have a circular mouth surrounded by barbels, and to 

 possess no traces of jaws or paired fins, features which have led 

 to its being associated with the Cyclostomes. At the same time, 

 however, the caudal fin is large, with elaborately bifurcating 

 supports, the vertebrae are stout and ring-like, and the skull 

 complex, with what appear to be relatively enormous auditory 

 capsules. On the whole, its affinities with the Lampreys are 

 non proven^ and Palaeospondylus must be regarded as one of the 

 puzzles of palaeontology as yet unsolved. 



Before passing to the Selachians proper, another interesting 

 group of extinct fishes is worthy of consideration. Their 

 remains occur in Devonian and Lower Carboniferous strata, 

 and their anatomy has been recently elucidated by Dr. Stensio. 

 They were formerly regarded as archaic Lung-fishes, but it is 

 now certain that they are in no way related to the Bony Fishes, 

 but represent an independent oflfshoot from the main stem of 

 primitive Sharks, a stem which became extinct during the early 

 part of the Carboniferous period. They might well be placed 

 in the class of Selachians, but it is perhaps better to regard 

 them as constituting a distinct class, the Placoderms, of which 

 two sub-classes may be recognised, Arthrodira and Antiarcha. 

 These fishes have a stout external skeleton of bony plates, a 



