192 



A. S. PACKAED, JR, ON THE 



Lawrentian, 



Archlcarls. 



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

 organized Paradoxides, etc., we find in the Lingula figures the remains of a species of 



jj Nebaliadse, the Hymenocaris vermi- 



? ^ . Cauda. Mr. J. W. Salter ^ who was 



the first author to draw attention to 

 the close relation of the fossil genera 

 Hymenocaris, Ceratiocaris, Peltocaris, 

 Dictyocaris, etc., to Nebalia, has given 

 us a series of sketches showing graphi- 

 cally the geological succession of this 

 group and the Estheriadte. Hymen- 

 ocaris, which Salter regards as " the 

 more generalized " type, lived during 

 the primordial period; Peltocaris, and 

 Discinocaris (Woodward) characterize 

 the Lower Silurian pei'iod ; Ceratiocaris the Upper ; Dictyocaris 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 Palaeozoic species were gigantic in size, 

 some being about a foot or more (the carapace of Dithyrocaris pholadomya Salter, being 

 seven inches long) in length, whUe our recent Nebalia is less than an inch in length. 



The Potsdam sandstone also contains the remains of a third grand division of Entomos- 

 traca, 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 '•'Classen und Ordnungen der Thierreichs" ) 

 suggests that the Lernseans might have infested Paleozoic fish, and on general grounds we 

 shoidd think that they probably extended as far back as the primordial zone, inasmuch as 

 highly developed Trilobites and Ostracodes appear there. Another argument is the 

 interesting discovery made in 1865, by Mr. Woodward, of the Cirripede, Turrilepas 

 Wrightii, from the Wenlock Limestone and Dudley Shale of the upper Silurian formation. 

 Previous to this, accoi'ding to Woodward, "the oldest known Cirripede was the Pollicipes 

 Mhceticus from the Rhoetic beds of Somersetshire"; while the type is not uncommon in the 

 Cretaceous, and has flourished from that period to the present. 



Of the Merostomata the oldest group is the Eurypterida ; the Xiphosura not dating 

 beyond the Lower Carboniferous. The Eurypterids have not been found below the Upper 

 Silurian (Lower Helderberg in America), and the aberrant forms Hemiaspis, Bunodes, 

 Pseudoniscus and Exapinurus are Upper Silurian forms. 



Among the Xiphosura, Cyclus, the lowest form, is found in the Carboniferous, and ranges, 

 according to Woodward, as far up as the Permian. In the same period occur Bellinurus, 

 Prestwichia and Euproops, being in this country found in the lower part of the true coal 

 measures, and found associated in the same beds with Ceratiocaris, Eurypterus (Anthra- 

 conectes) and certain Isopoda and Macrurous Decapoda (Anthrapalseraon). The genus 



*0n Peltocaris, a new genus of Silurian Crustacea. Quart. Journ. Geol. Society, London, vol. xix. 1863, p. 87. 



