190 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



Sub-Class TRILOBITA 



NOTES ON SOME APPENDAGES OF THE GENERA NEOLENUS 

 AND PTYCHOPARIA 



We now have from the Burgess shale specimens of a Middle 

 Cambrian trilobite (Neolenus serratus (Rominger)), showing the 

 character of its antennae, legs, branchiae, and caudal appendages, and 

 another (Ptychoparia cordillercz (Rominger)) with a full series of 

 branchial appendages. Also there are five Ordovician forms, three 

 from the Trenton limestone (Calymene senaria, Ceraurus pleiire- 

 xanthemus, and Asaphus platycephalus) and two from the Utica 

 shale (Triarthrus hecki. and Trinucleus concentricus) , which pre- 

 serve the antennae, legs, and branchiae. 



A review of these shows a surprising uniformity of structure of 

 the antennae, legs, and branchiae in genera separated by great inter- 

 vals in the stratigraphic series, and distinguished by marked varia- 

 tions in the external form of the dorsal shield. 



At first ^ I was inclined to consider that the trilobite was a highly 

 organized crustacean approaching the merostomes, but with the data 

 now available I join with Burmeister * and Bernard * in considering 

 that the trilobite is more closely related to the branchiopod crusta- 

 ceans. Burmeister wrote in 1843 • * 



The trilobites were a peculiar family of Crustacea, nearly allied to the 

 existing Phyllopoda, approaching this latter family most nearly in its genus 

 Branchipiis, and forming a link connecting the Phyllopoda with the Poecilo- 

 poda. 



In order, however, to estimate fairly the affinity of the trilobites with the 

 Phyllopoda, we must not lose sight of the important fact that the trilobites 

 differ not only from the Phyllopoda, but from all other existing families of 

 Crustacea in the varying numerical proportion of their thoracic rings; a pecu- 

 liarity neither exhibited at present as a characteristic of any natural family 

 among the Crustacea, nor in any of the heterogeneous Articulata. This 

 peculiarity occurs, it is true, among the Aspidostraca (a group of the second 

 great division of the crustaceans), but only in a modified form, the difference 

 in the numerical proportion being always reducible to one fundamental num- 

 ber. This law is apparently not observed in the case of the trilobites. 



It would seem then that the relation existing between the trilobites and the 

 existing Crustacea is one rather of analogy than affinity, so that the whole 

 group may be considered as a separate division, corresponding with the 

 Aspidostraca in the formal variation presented from the typical character, but 



^ The Trilobite, New and Old Evidence relating to its Organization. Bull. 

 Mus. Comp. Z06I. at Harvard College, Vol. 8, 1881, pp. 208-211. 

 *The Organization of Trilobites. London, 1846, p. 46. 



* The Systematic Position of the Trilobites. H. M. Bernard, Quart. Journ. 

 Geol. Soc, London, Vol. 50, 1894, pp. 411-432; and Vol. 51, 1895, pp. 352-359- 



* Burmeister, " The Organization of Trilobites," 1846, pp. 46-47. 



