188 A. S. PACKARD, JR., ON THE 



Woodward/ who has " discovered upon the eroded upper surface of one of these, not only 

 the hypostoma exposed to view, but also three pairs of appendages, and what he believes 

 to be the palpus of one of the maxillae." 



Glaus, in his " Grundziige der Zoologie," 1868, thus characterizes the order Poecilopoda: 

 " Crustacea with a thick-shelled, arched cephalothorax, flat, shield-like abdomen, and a 

 moveable caudal spine. Gills on the abdominal feet." This definition, now that we have 

 indications that the Trilobites probal>ly had thoracic ambulatory feet, and, in all probability, 

 leaf-like appendages on the abdomen (pygidium), will apply about as well to Trilobites as 

 to Limuli. We would suggest that while the differences between the two groups are very 

 marked, that they are yet probably only ordinal, and that the two groups fall naturally 

 into the subclass Branchiopoda,^ which Glaus (who considers it as an order) characterizes 

 thus : " Grustacea of elongated, often clearly articulated bodies, for the most part with 

 shield-shaped, cloak-like, or a bivalve duplication of the integument, with at least four pairs 

 of leaf-like, foliaceous, swimming feet." This definition does not exclude the Merostomata, 

 and probably, barring the number of pairs of abdominal feet, includes the Trilobites. The 

 orders may be arranged thus in the ascending order : — 



1st order. GLADOGERA. " Small, compressed Branchiopoda [' Phyllopoda ' Glaus], 

 with non-articulated bodies, mostly with a bivalved shell and free head extended forward, 

 with large oar-like antennae (ruderarme), and four to six pairs of swimming feet. A single 

 family, Daphniadce. (Sida, Daphnia, Polyphemus, etc.)" (Glaus.) No metamorphosis, 

 young resembling the adults. 



2d order. MEROSTOMATA. Gigantic Branchiopoda, with the head covered by a large 

 or medium sized shield, antennae forming ambulatory limbs ; four to six ambulatory, usually 

 forcipated maxillipedes ; abdomen with not over six pairs of lamellate swimming feet, 

 most of which bear gills ; and ending either in a caudal spine, or flattened telson. A pair 

 of compound and a pair of simple eyes. 



Suborder 1. Xiphosura. Body trilobate in the young (Limulus), and often in the 

 adult. Cephalic shield very large, convex, semicircular, bearing (when known) six pairs 

 of forcipated. ambulatory, simple, cephalic appendages, of which the five posterior pairs are 

 provided with coxal spines for chewing the food ; a true hypostoma. Metastoma forming 

 two separate tubercles. Abdomen 8-9 jointed, with segments more or less distinct, the 

 terminal one forming a large spine. A shght metamorphosis ; the young resembling the 

 adult. Eggs not borne about by the parent during incubation. Three ? families. 1. Limu- 

 lidaj (Euproops, Prestwichia, Bellinurus, Limulus). Cyclus " is perhaps the type of a second 

 family ; and the two genera, Exapinurus and Pseudoniscus, possibly, are types of a third 

 family. 



^ "Note on the Palpus and other Appendages of Asaphus (1. c, p. 5, footnote) Crustacea, I think we are wan-anted in 

 from the Trenton limestone, in the British Museum." By regarding it as a member of the Xiphosura. He gives de- 

 Henry Woodward, Proceedings of the Geological Society scriptions and figures of nine species, of which Cijclus Hark- 

 of London, May 11. Reported in Nature for June 2, 1870. j?es,«' Woodward, and C. /oiiesmnus Woodward (fig. 36,6, 



2 Claus uses the term Phyllopoda for the order, but Branch- dorsal aspect of carapace ; a, side view of the same ; both mag- 



iopoda of Lamarck (1801) seems to have the priority, accord- nified four times), are fair examples. The body is hem- 



ing to Agassiz's Nomenclator Zoologicus. ispherical, and afibrds no indication of a distinct abdomen', 



8 From the evidence presented by Mr. H. Woodward the margin of the body may be compared with the margin of 



(Contributions to British Fossil Crustacea, Geol. Mag., Dec, the phield of Limulus. We can also detect the median 



1870, VII, p. 1) as to the structure and aflinities of tliis most "cardiac" lobe of "Woodward lower down the lateral, and 



interesting and singular genus of Carboniferous and Permian below the marginal lobe, corresponding to the three lobes of 



