PACKARD.] GENEALOGY OF CRUSTACEA. A19 
of these Crustacea. There are no marine allies of the Phyllopods. 
Moreover all the fossil forms appear to have been fresh-water forms, 
their remains occuring in fresh-water strata. No fossil Phyllopods 
have occurred as yet previous to the Devonian Period. 
The difficulty is (and this is a point apparently overlooked by Fritz 
Miiller, Dohrn, Claus, and Balfour) to account for the origination of the 
Phyllopods at all from any marine forms. The only explanation we can 
suggest is that the Phyllopods have arisen through Limnetis directly 
from some originally marine Cladocerous type like the marine forms now 
existing, such as Hvadne. We imagine that when a permanent body of 
fresh water became established, as, for example, in perhaps early Silu- 
rian times, the marine forms carried into it in the egg-condition, possi- 
bly by birds or by high winds, hatched young, which, under favorable 
conditions, changed into Sida, Moina, and Daphnia-like forms. The 
Cladocera are, then, probably the more generalized forms, from which 
the Phyllopods, at this time and probably ever since Devonian times, 
par excellence a fresh-water assemblage of forms, took their origin. This 
view, it seems to us, accords with the well known facts in the biology 
and paleontology of these forms. 
The view which we believe Dohrn entertains, and to which Mr. Bal- 
four gives some support (though Claus opposes it), that the Ostracoda 
may have decended from ancestors with a larger number of appendages 
than they have at present, appears to us to be negatived by the fact 
that their valves are so abundant in the lowest paleozoic rocks. The 
type appears to have persisted and to have remained unchanged from 
the Potsdam Period to the present day, and is more marine than fresh 
water. So close do the lower Cladocera approximate to Cypris that the 
transformation of an Ostracode! into a Cladoceran, and a Cladoceran 
into a Phyllopod, is much more easily imaginable than a hypothetical 
Protophyllopod ancestor for the Phylopods. 
All this clears the way for the view that the Malacostraca had an in- 
dependent origin from some Nauplius, through, we will admit, some an- 
cestral Protophyllopod form which was succeeded by a protozoéa, and 
finally a zoéa, the ancestor of the existing Decapods as a whole; and it 
also leaves open a field for the independent evolution of the Phyllocari- 
dan type, composed of gigantic Nebalia-like Silurian forms, which also 
have originated at a much earlier date than the Decapods, and have 
held somewhat the same relations to the Decapods as the Eurypterida 
did to the Limuli. 
In conclusion, therefore, we consider the Phyllopods as a whole, 
especially the Apodide and Branchipodide, to be a comparatively recent, 
highly specialized group, which were developed under exceptional bio- 
logical conditions in bodies of fresh water, and which, as in Apus, show 
that this branch of the Crustacean genealogical tree has culminated. 
The irrelative repetition of the segments and appendages (in Apus) 
gives evidence that the type, so far from being ancestral, is one com- 
paratively modern, specialized, and fully worked out. 
1 Balfour remarks that ‘‘the independent origin of the Ostracoda from the maia 
Crustacean stem seems probable.” Page 424, 
