218 PALAEONTOLOGY. 



pital condyle or condyles in Archegosaurus, as in Lepido- 

 siren ; the presence of labyrinthic teeth in Archegosaurus, 

 as in Lepidosteus and Lcibyrinthodon ; the large median 

 and lateral throat-plates in Archegosaurus, as in Megalichthys 

 and in the modern Arapaima and Lepidosteus ; — all these 

 characters point to a great natural group or series, shewing 

 the gradations of development which link and blend together 

 fishes and reptiles within the limits of such group. The sala- 

 mandroid (or so-called " sauroid") Ganoids — Lepidosteus and 

 Polypterus — are the most ichthyoid, the true Labyrinthodonts 

 are the most sauroid of the group. The Lepidosiren and 

 Archegosaurus are intermediate gradations, one having more 

 of the piscine, the other more of the reptilian, characters. 

 The Archegosaurus conducts the march of development from 

 the ganoid fishes to the labyrinthodont type, the Lepidosiren 

 to the perennibranchiate type. Both illustrate the artificiality 

 of the supposed class-distinction between fishes and reptiles, 

 and the unity of the " Hsematocrya," or cold-blooded Verte- 

 brata, as a natural group. There is nothing in the known 

 structure of the so-named Archegosaurus or Mastodonsaurus 

 that truly indicates a belonging to the saurian or crocodilian 

 order of reptiles. The exterior ossifications of the skull and 

 the canine-shaped labyrinthic teeth are both examples of the 

 salamandroid modification of the ganoid type of fishes. The 

 Ganocephala and Labyrinthodontia characterize the transitional 

 period between the palseo- and mezo-zoic epochs. 



Order 3. — Ichthyoptekygia.* 



The bones of the head still include the supplementary " post- 

 orbitals" and " supra-temporals," and there is a "fora- 

 men parietale ;" but there are small temporal and other 

 vacuities between the cranial bones, a single convex 

 occipital condyle, and one vomer which is edentulous. 



^ * l%dvs, a fish; vrripv^, a fin. 



