PACKARD.] 



GEOLOGICAL SUCCESSION OF CRUSTACEA. 



\61 



species is Permian), Griffithides, and Brachymetopus being tlie sole rep- 

 resentatives of the type which prevailed so extensively during the Si- 

 lurian. 



Geological succession of the Crustacea. 



Branchiopocla. 



Quaternary 

 Tertiary 



Mesozoic 



Carboniferous 



Devonian 



Silurian 



Laurentian 



Protonauplius. 



Protocyclus. 



Simultaneously with the appearance of the larva-like Agnostus, and 

 the more highly organized Paradoxides, &c., we iind in the Lingula flags 

 the remaius of a species of Fhyllocarida, 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 Estheriada3. 

 Hymenocaris, which Salter regards as "the more generalized" type, 

 lived during the inimordial 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. ISTo Mesozoic member of the family has 

 yet been discovered, but as there are several species of i^Tebalia 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 ISTebalia 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. 



E'o fossil Copepoda have yet been discovered, but we should scarcely 

 wonder at this, owing to their soft bodies. Gerstaecker (Bronn's 



