FOSSILS AND PEDIGREES 353 



and Guitar-fish (Rhinobatus), as well as the Comb-toothed 

 Sharks (Hexanchidae), Bull-headed Sharks (Heterodontidae) , and 

 some of the Dog-fishes (Scyliorhinidae) , date back to the Jurassic 

 period. Indeed, so little have some of these changed in course 

 of time, that well-preserved remains of Squatina or Rhinobatus 

 from the Lithographic Stone (Jurassic) of Bavaria are almost 

 indistinguishable from their descendants of the present day. 

 The curious Elfin Shark {Scapanorhynchus) was first found in a 

 living condition in deep water off Japan in 1898, but the same 

 genus had long been known from fossil remains in the Cretaceous 

 rocks. An interesting Shark (Protospinax) has recently been 

 described from the Jurassic, and seems to bear the same relation 

 to the Saw Sharks (Pristiophoridae) as does the Guitar-fish 

 {Rhinobatus) to the ray-like Saw-fishes [Pristidae). It has a 

 pointed rostrum, not yet armed with teeth at the sides, but, 

 apart from the more primitive character of the median fins, it 

 is very like the Saw Sharks, to which it was probably ancestral. 



The Holocephali also date from the Triassic period, and reached 

 their zenith during the Cretaceous and Eocene. Some of the 

 Devonian and Carboniferous ichthyodorulites may have 

 belonged to ancient members of this sub-class, but this is by 

 no means certain. The comparatively few existing Chimaeras, 

 etc., may be looked upon as the degenerate descendants of an 

 important group, the members of which were once numerous 

 and diverse. The living forms rarely exceed a length of three 

 or four feet whereas the extinct genus Edaphodon attained to 

 relatively gigantic proportions. 



As in the case of the Selachians, little is known of the actual 

 beginnings of the class of Bony Fishes (Pisces), although it is 

 fairly certain that these arose as an offshoot from the primitive 

 Sharks at some time during the Silurian period. A modern 

 taxonomist divides the Pisces into three main groups or sub- 

 classes: Palaeopterygii (ancient fins), Crossopterygii (fringed fins), 

 and Neopterygii (new fins). The diflferences between them are 

 of a technical nature, involving the form of the skeleton and 

 scales and the structure of the fins. It may be noted that, as 

 far as the living representatives are concerned, the three groups 

 are vastly different in size : the first and second together include 

 some ten existing genera with less than fifty species all told, 

 whereas the Neopterygian sub-class contains numerous genera 

 and probably at least fifteen thousand species. In the past, 

 however, at a time when the modern Bony Fishes had not yet 

 come into existence, the first two sub-classes were spread all 



