DEVONIAN ANIMALS. 



339 



Ganoids), or true Ganoids, and Placo -ganoids (plate-ganoids), or Placo- 

 derms. The former are covered, like modern Ganoids, with bony, en- 

 ameled scales, which may be close-fitting, rhomboidal (Figs. 432-434), 

 or imbricated (Fig. 431). The latter are covered more or less com- 

 pletely with broad, immovable plates. The Placoderms are entirely 

 characteristic of the Devonian (and Upper Silurian), and become extinct 

 after that time. The Lejndo-ganoids have continued in diminishing 

 numbers and different species, genera, and families, to the present day. 

 One family of these is very characteristic of Devonian — viz., Cros- 

 sopterygians (f ringed-limb), so called because the limbs seem to come 

 out from the body into the fin like a true leg. All the strangest and 

 largest forms — such as the Cephalaspis, the Coccosteus, the Pterych- 

 thys, the Dinichthys, and the Titanichthys, are Placoderms. They 

 were heavy, sluggish, uncouth animals, relying for safety rather upon 

 protective armor than upon swiftness. 



Nearest Allies among Existing Fishes. — The Placoderms have no 

 close allies among living fishes. Some have imagined the sturgeon to be 

 distantly allied ; and Dr. Newberry finds some affinities in the Lepi- 

 dosiren with the Dinichthys. They were probably a generalized type 

 connecting Ganoids and Placoids. The Lepido-ganoids, however, still 

 have living congeners, which throw light upon their structure (Figs. 

 437-441). Among the nearest allies are the Lepidosiren and Protop- 

 terus of South American and African rivers, the Ceratodus of Austra- 

 lian rivers, and the Polypterus of the Nile. It will be noted that these, 

 especially the first two, have almost veritable legs instead of paired 

 fins. The Ceratodus especially is a living Crossopterygian. It is well 

 to note also that the Lepido- 

 siren is the most reptilian or 

 rather amphibian of all fishes, 

 and next in this respect comes 

 Ceratodus. These two have a 

 three-chambered heart and a 

 tolerably good lung and nos- 

 trils, and breathe air as well as 

 water, like many amphibians. 

 They also have cartilaginous 

 skeletons, like Devonian fish- 

 es. Less near allies are found 

 in the gar-fish (Lepidosteus) 

 and the mud-fish (Amia) of 

 our Atlantic and Gulf rivers. 

 These also supplement their gill-breathing with a little air gulped 

 down into their vascular air-bladder from time to time. 



Fig. 443.— Dental Plate of Cestracion Phillippi. 



The nearest congener of Devonian Placoids is found in the Cest 



ra- 



