DEVELOPMENT OF THE STUBGEON: 427 



in the garpikes. The oviducts communicate with the ure- 

 ters as in the sharks and amphibians. The different modifi- 

 cations of Ganoid structure may be observed in the examples 

 ■of the different orders. 



Many of the Ganoids of the Upper Silurian and Devonian 

 Tocks belonged to the groups Cephalaspidm and Placoder- 

 mata. In the Cephalaspids, represented by the singular 

 Cephalaspis Lyellii of Agassiz^ the broad head was covered 

 by a single semi-circular plate, vs^ith large orbits above, the 

 mouth being below. The pectoral fins were rayless folds of 

 the skin ; the body behind the head was covered with rhom- 

 Tjoidal scales, and provided with a dorsal fin. The Pteraspis 

 liad a head-shield composed of seven pieces. Among the 

 Placoderms, PtericMhys had a j)lated head half as long as 

 "the body, the tail short and scaled. These fishes, the earliest 

 known Vertebrates, were bottom-feeders. ISTothing is known 

 .as to the nature of their jaws or teeth. 



Order 1. Chondroganoidei. — In these Ganoids the dorsal 

 chord is not ossified ; the skull is cartilaginous, covered with 

 membrane-bones ; they are either toothless or with small 

 teeth. The skin is naked as in the paddle-fish, or protected 

 as in the sturgeons with very large, bony, solid plates. The 

 -sturgeons have the snout long and pointed, with the mouth 

 underneath, and toothless. Acipenser sturio Linn, is the 

 common sea-sturgeon of our coast, ascending rivers. The 

 shovel-nosed sturgeon, ScapMrhynchops platyrhynchushas, a 

 spade-like snout. It inhabits the waters of the Mississippi 

 Valley. Salensky has studied the embryology of the Russian 

 sturgeon. The freshly-laid eggs are two millimetres in di- 

 ameter, the yolk undergoes nearly total segmentation, thus 

 connecting most Vertebrates in which the eggs only partially 

 segment, with the Amphioxus, lampreys, and amphibia, in 

 which segmentation is total. The skeleton is developed 

 much as in the Elasmobranchs. The sheath of the noto- 

 chord develops in three weeks after hatching. At the 

 end of the third week the upper and lower vertebral arches 

 appear, arising as in other fishes. The skull is indicated in 

 two or three weeks after hatching. 



