* 
and on some characteristics of the Reptilian Birds. 317 
intermediate or Jchthyoid one of Amphibians—the young of frogs 
and salamanders and other included species — gills like 
fishes, besides some additional fish-like peculiaritie 
The parallelism between the three classes, Mainitile ‘Birds — 
and cg ae is thus complete 
(4.) Fishes have no class of Vertebrates below them, so that 
an inferior hemitypic division is not to be looked for. It might 
be suspected that the intermediate group in this case would be 
one between Fishes and the lower subkingdoms either of Mol- 
—_ or of Articulates; but none such exists. The lowest fish, 
Amphioxus, is as distinctly a Vertebrate as the highest, and 
as Mollusk or Articulate exhibits any transition towards a ver- 
tebrate structure, 
There are, however, hemitypic Fishes; but their place is to- 
wards the top of the class instead of at its bottom. Ganoids con- 
stitute one group of this kind, between Fishes and Reptiles, as 
intermediate in character between Fishes and the allantoidian 
Vertebrates. Moreover, Miller long ago observed the relation 
of the Sharks to the Mammals in having a vitelline placenta, by 
which the embryo draws nutriment from the parent, as does t 
mammalian fetus by means of its allantoidian placent 
Ganoids ot Selachians are, thus, two hemitypie groups in 
the class of 
The chan ‘of grand divisions is then as follows :’ 
LE 
A. Typical Mammals, 
B. Hemitypic Mammals. 
or Odrocors. 
IIL. 
A. Typi A. Typical or true Reptiles. 
B. He emitypic Bind B. Beanies! Rapisles 
or 
MPHIBIAN 
LY. 
A. Hemitypic Fishes, B. Hemitypic Fishes, 
or SELACHIANS. or GANoIDs, 
C. AE Fishes, 
or Peli 
One of the groups of hemitypic Fishes ae directly towards 
Reptiles, and the other towards the three higher nae of Ver- 
tebrates collectively, but especially Mammals and Bi 
* It is here Ost. d to Marsu oh ltt 
has. ne aime rpaoids, and Am pasion. Oétocoid is 
y the Greek form of the term semi-oviparous. 
