344 A HISTORY OF FISHES 



must be at hand, capable of appreciating the fossil, and pre- 

 serving it for study when discovered." 



The geological record has so far provided no evidence as to 

 the origin of the three vertebrate classes here grouped together 

 as fishes, and at the time when fish-like fossils first made their 

 appearance in the rocks the Marsipobranchs, Selachians, and 

 Bony Fishes are not only already differentiated from each other 

 and firmly estabHshed, but are represented by a number of 

 diverse and often specialised types, a fact that suggests that each 

 of the classes had already enjoyed a respectable antiquity. It 

 will be convenient, therefore, to consider the fossil history of 

 each separately, commencing with the Marsipobranchs as 

 being admittedly the most primitive. 



In the Marsipobranchs (pouch-gills) the gills are contained 

 m a series of separate muscular pouches, which expand and 

 contract during respiration, and are quite unhke those of any 

 other fish {cf. p. 37). This character, coupled with the com- 

 plete absence of jaws and of gill-arches, serves to distinguish 

 them from both Selachians and Bony Fishes. It had formerly 

 been supposed that these characters represented secondary 

 modifications brought about by the highly specialised, semi- 

 parasitic habits of the existing Gyclostomes, but the past history 

 of the group, recently correctly interpreted for the first time, 

 eflfectually disposes of this view, and places beyond all doubt 

 that the dififerences between the Marsipobranchs and other 

 vertebrates in the structure of the mouth and gills are truly 

 fundamental. It must be understood, of course, that the fossil 

 remains furnished by the Silurian and Lower Devonian rocks 

 provide few clues as to the fines of descent of the existing forms, 

 these archaic forms being quite as speciafised in many respects 

 as are their descendants living to-day. At the same time, 

 however, they are of very great interest, and show in their 

 anatomy undoubted evidence of descent from the same stock. 



These fossil Marsipobranchs, the oldest fish-fike vertebrates 

 known, were formerly grouped together with other superficially 

 similar forms under the name of Ostracoderms, but three main 

 groups are now recognised, each ranking as a distinct sub-class, 

 the fourth sub-class, the Gyclostomes, being reserved for the 

 existing forms. The Anaspida have a typical fusiform body, 

 covered with series of small scale-like plates, the head being 

 armed with numerous plates of a similar nature, but less regularly 

 arranged (Fig. 123A). Immediately behind the head on either 

 side IS a row of gill-openings, and special plates and small spines 



