AGASSIZ. NATURAL HISTORY OF THE UNITED STATES. 353 
separate group, its several members taking their rank next those 
undoubted zooids which in structure they most closely resemble. 
According, therefore, to the views now here laid down, the 
proper value of the primary divisions of Hydrozoa will be as 
follows :•— 
Class HYDPOZOA. 
Sub-classes. 
I. Hydrotda, Johnst. 
II. Siphonophora, Eschsch. 
III. Discophora, Eschsch. 
Orders. 
1. Hydrida. 
2. Tubularida or Corynidct. 
3. Sei'tularidci. 
S 4. Calycophorida. 
\ 5. PJiysophorida. 
. 6. Medusida. 
Lucernaridae. 
iEginidae. 
Charybdeidse. 
Monostomidae. 
Ehizostomidae. 
Excepting its introductory chapter, which we have noticed at so 
much length, Part III. essentially includes— 
1. Original observations on various North American forms belong¬ 
ing to the order. 
2. Critical remarks on other forms more or less closely allied to 
these. 
3. A “ Tabular view of the Discophorae known at present,” and 
4. A short section on the Geographical Distribution of the Disco¬ 
phorae. 
The original observations have reference to the following 
species 
a. Aurelia Jlamdula. 
1). Cyanaea arctic a. 
c. JPelagia cyanella. 
d. Stomoloplius melcagris. 
e. Polyclonia frondosa. 
The first two of these, more particularly Aurelia, are described 
with very great minuteness of detail, the majority of the beautiful 
plates in Yol. III. being devoted to their representation. These two 
Medusae , in their free sexual state so dissimilar, are, it is well known, 
wonderfully alike in their polypoid or hydra-tuba stage. Professor 
Agassiz has taken considerable pains not to confound these two 
kinds of hydra-tubae with one another. We would almost go so far 
as to say that his plates have a greater value than his descriptions, 
though both taken together illustrate the structure and develop¬ 
ment of the higher Medusae in a more complete manner than any 
other monograph yet published. It is indeed true that, by com¬ 
bining and correcting the several essays of Sars, Siebold, Ehrenberg, 
and a number of other observers, most of the information which 
