PALEOZOIC FISHES. 301 



armor, may be conveniently recognised : the osseous plated ganoids, 

 as represented by the sturgeon ; the enamel rhomb-plated ganoids, 

 typified in the American alligator-gars (Lepidosteus) and the Afri- 

 can Polypterus ; and the cycloid scaled ganoids, exemplified in 

 the American Amia, which may in a measure be said to connect 

 these fishes with the herring among the teleosts. None of these 

 types appear to be represented in the Silurian period, unless, in- 

 deed, the group of the bucklered ganoids, to which the ancient 

 Scaphaspis Ludensis, and the greater number of the Devonian 

 forms (Cephalaspis, Pteraspis, Pterichthys, Coccosteus) belong, be 

 considered to be nearly related to the sturgeons. This relation- 

 ship, however, requires further demonstration before it can be 

 accepted as a fact ; indeed, it has recently been attempted to show 

 that some of these most ancient ichthyoid forms e. g., Pterichthys 

 are not fishes at all, but members of what may, perhaps, be con- 

 sidered to be a degenerated group of the lower Vertebrata, the 

 tunicates. The almost total obliteration of the bucklered type of 

 ganoid with the close of the Devonian period is very remarkable, 

 and has not yet received a satisfactory explanation. It has been 

 conjectured by some that these fishes early withdrew to fresh water, 

 and that, in the absence of fresh-water deposits of any magnitude 

 in the period succeeding, they have necessarily left behind but 

 scanty traces of their existence. It must be confessed, however, 

 that this explanation is more in the nature of an assumption than 

 anything else, and has but little positive to bear it out. In how 

 far the Devonian fishes were of a fresh- water habit still remains to 

 be determined, but it seems more than probable that they were 

 largely of this, or at least of a brackish-water, character. The re- 

 mains of the sturgeon are not known prior to the Eocene period, 

 although a direct forerunner (Chondrosteus), uniting this family 

 with the Spatularidse, occurs as low down as the Lias. 



The non-bucklered ganoids of the Devonian period (Holop- 

 tychius, Glyptolepis, Dipterus, Osteolepis, Diplopterus) belong 

 principally to the type of the fringe -finned or crossopterygian 

 Polypteri, which effect a passage to the lung-fishes (Dipnoi). This 

 series, which is represented by both scaled and plated forms, is 

 continued into the Carboniferous period (Rhizodus, Dendrodus, 

 Megalichthys, Coelacanthus) ; but here, as in the succeeding Permian 

 period, they are already largely replaced by the lepidosteoid type 



