502 Prof. J. D. Dana on the 



disk towards the outer margin. On tlie posterior wings the 

 metallic colouring at the hase of both wings and in the discal 

 band across the posterior wings is more bluish and less 

 golden green, while the black velvety spots in the band are 

 more extended inwardly ; the outer edge of the band is further 

 from tiie outer margin and more convex between the veins. 

 Both wings are broader and more rounded at the apex. 



Expanse of wings If inch. 



flah. New Britain. 



LVI. — Observationfi on the Derivation and Homologies of 

 some Articulates. By JamkS D. Daxa *. 



The term Articulates is used here in preference to Arthropods, 

 because the latter group is believed to be not a natural one, 

 Crustaceans and Insects being less closely related to one 

 another, as indicated beyond, than Annelids and Insects. 



Derivation of Limuloids and Crustaceans. — As has been 

 suggested by Lankcster, it is probable that all the Articulates 

 are successional to the Rotifers. There is reason for believing 

 further that the types of Annelids, Crustaceans, and probably 

 that of Limuloids had their independent Rotifer origin. 



The Nauplius, or larval form of a Crustacean, shows, by its 

 having but three pairs of limbs (two besides an antennary 

 pair), that the type is not successional to a many-jointed 

 Annelid, but rather to some Pedalion-VikQ Rotifer. The 

 discoveries of Prof. C. E. Beecher announced in the preceding 

 and earlier numbers of this * Journal ' leave no doubt that the 

 Trilobites are multiplicate Isopod Crustaceans, precursors of 

 the normal Isopods, as the true Phyllopods, also multiplicate 

 species, were precursors of the Decapods f- 



The Eurypterids, the early form of the Limuloids, are 

 related to Crustaceans in number of body-segments, it leing 

 19, as in the Tetradecapods, and in the tact that 18 of these 19 

 segments pertain to the thorax and abdomen. But the wide 

 distinction exists that the Eurypterids have no thoracic or 



* From the ' American Journal of Science,' May 1804. pp. ."^^o-SiO. 



t In the Author's ' Report on the Crustacea of the AVillies Explorinft 

 Expedition,' the Rotifers are made the lowest subdivisions of Crusti\cea 

 (p. 1408), and tlie Trilobites are placed, with a query, in the subilivision 

 of Tetradecapods, as multiplicate forms under the type. In the text 

 above the expression true IMiyllopods is used, because most of the so- 

 called Phyllopods of tiie Palaeozoic exliibit, in the specimens, no evidence 

 that they are nniltiplicate, that is, have an excessive or abnormal number 

 of body-sefrmeiits or ajipendap-ofl. 



