34 
spiral riband or ridge, very variable in its proportions, wMcli 
lines tbe lower part of the intestinal tube, and in many cases 
its wbole length. Miiller speaks of it as not known in all 
Ganoids, Lepidosteiis being an exception so far as he knew. 
Subsequent dissections have demonstrated the presence of a 
rudimentary but strictly homologous structure in this genus 
also, so that we may now regard the spiral valve as a fresh char- 
acter of Ganoids, distinguishing them from all Teleostei, asso- 
ciating them more closely with Elamobranchii and Le^ndosiren. 
Some of the inconstant features of the Ganoids are avail- 
able for subdivision of the order. Amia, Polypterus, and 
Lepidosteus are covered with scales and have a bony skeleton ; 
the Sturgeons, Scaphirhynchiis and Polyodon are naked or 
only partially covered with osseous scutes, while their endo- 
skeleton is incompletely ossified, especially in the skull. These 
points of difference define the two sections — the Holostei and 
the Chondrostei of MuUer. As subsidiary and merely generic 
characters the varying structure of scales and fins has value. 
Amia is cycloid; Polypterus and Lepidosteus ganoid in the 
sense of Agassiz. Lepidosteus and the Chondrostei have fulcray 
or spines attached to the edge of the fins ; Polypterus and 
Calamoichthys have fins without fulcra. Most of the Ganoids 
have abdominal ventral fins ; Calamoichthys has no ventral 
fins at all. 
Miiller's discoveries were sufficient to define the Ganoid 
order, to subdivide it into two natural sections, and to arrange 
under these with just and precise definitions all the then 
known forms. It now remains to narrate briefly the progress 
of discovery since 1844, and to discuss the position of the 
Ganoids in the class Pisces as disclosed to us by the more 
complete knowledge which further investigation has furnished. 
Before entering upon this narrative of later research, let 
us turn back for a moment to the discovery in 1839 of the 
real structure and zoological significance of Lepidosiren. 
