DEVELOPMENT OF THE STURGEON. 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 
rocks belonged to the groups Cephalaspide and Placoder- 
mata. In the Cephalaspids, represented by the singular 
Cephalaspis Lyellai of Agassiz, the broad head was covered 
by a single semi-circular plate, with 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- 
boidal scales, and provided with adorsal fin. The Pteraspis 
had a head-shield composed of seven pieces. Among the 
Placoderms, Pterichthys had a plated head half as long as 
the body, the tail short and scaled. These fishes, the earliest 
known Vertebrates, were bottom-feeders. Nothing is known 
as to the nature of their jaws or teeth. 
Order 1. Chondroganoidet.—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, Scaphirhynchops platyrhynchus has 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. 
