PACKARD. ] GEOLOGICAL SUCCESSION OF CRUSTACEA. 361 
species is Permian), Griffithides, and Brachymetopus being the sole rep- 
resentatives of the type which prevailed so extensively during the Si- 
lurian. 
Geological succession of the Crustacea. 
Branchiopoda. 
——- + =. 
3S & 
: & : 
3 = 5 
Bibi faba be va UP ER ea ee £ 
toad F Sie 8) Heniies E F 
=] Oo = a St e SS ° ~~ 
= re 5 qa ne} SS CS) S o°0 ~ a= 
a Ps] = o og SS a ae") DQ 2 
3 q = q SE) one a een ° ° 
>) we) ica] a tl ics es) [="I>) a ta 
S) i) Bs) H Se Q °.4 = 
A aA Ay Seoul iS}S) =| a 
Quaternary : 
Mertiany i. -cc--5- 5 5- 
Mesozoic) -sesss--41- 
Carboniferous .-...- 
Mevonian assess see 
SWirrmbin 5625 sebsaece 
Banrentian 0-22. - = 
Protonauplius. Protocyclus. 
Simultaneously with the appearance of the larva-like Agnostus, and 
the more highly organized Paradoxides, &c., we find in the Lingula flags 
the remains of a species of Phyllocarida, the Hymenocaris vermicauda. 
Mr. J. W. Salter, who was the first author to draw attention to the 
close relation of the fossil-genera Hymenocaris, Ceratiocaris, Peltocaris, 
Dictyocaris, &c., to Nebalia, has given us a series of sketches showing 
graphically the geological succession of this group and the Estheriadie. 
Hymenocaris, which Salter regards as “the more generalized” type, 
lived during the primordial period; Peltocaris and Discinocaris (Wood- 
ward) characterize the Lower Silurian period; Ceratiocaris the upper; 
Dyctyocaris the Upper Silurian and lowest Devonian; Dithyrocaris 
and Argus the Carboniferous. No Mesozoic member of the family has 
yet been discovered, but as there are several species of Nebalia now 
living in our seas, it is reasonable to suppose that the type has existed 
in an unbroken succession from primordial times until now. The Pa- 
leozoic species were gigantic in size, some being about a foot or more 
(the carapace of Dithyrocaris pholadomya Salter being seven inches 
long) in length, while our recent Nebalia is less than an inch in length. 
The Potsdam sandstone also contains the remains of a third grand 
division of Entomostraca, the Ostracoda; remains of Leperditia having 
been found in Canada, as well as the Lower Silurian of Europe. 
No fossil Copepoda have yet been discovered, but we should scarcely 
wonder at this, owing to their soft bodies. Gerstaecker (Bronn’s 
