190 



A. S. PACKARD, JR., ON THE 



hatched either with none, or one or two thoracic segments. Eggs probably deposited loose 

 in sand or mud. Seventeen femilies, with very nunaerous genera and species. 



4th order. Phyllopoda. Body with 10-64 distinct segments, naked, or partly sur- 

 rounded by a flat, shield-shaped or compressed shell, with from ten to sixty pairs of leaf- 

 like swimming feet, provided with well developed gills. Metamorphosis complete ; young 

 of the nauplius or zoea form. Family 1. EstheriadcB (Limnadia, Estheria, Limnetis). 

 2. Apusidm (Apus, Lepidurus). 3. Nehaliadce^ (Hymenocaris, Peltocaris, Ceratiocaris, 

 Dictyocaris, Dithyrocaris, Argus, Nebalia). 4. Branck'qyodidm (Branchipus, Branchinecta, 

 Streptocephalus, Chu'ocephalus, Artemia). It is not improbable that these four so-called 

 families represent suborders. 



We have previously indicated the close relationship of the Merostomata, both in their 

 adult and (as flxr as can be judged from the development of a single genus) larval stages, 

 with the Trilobites. To show still farther the close relation existing between the two 

 groups, I place their characters in opposing columns. 



Meeostomata. 



Body l.irge and massive, consisting of a head often 

 forming a broad cephalic shield, and abdomen indis- 

 tinctly or distinctly trilobate. 



Frontal doublure well marked. 



Compound eyes many facetted ; simple eyes. 



Hypostoma small, or large, squarish. 



Five to six pairs of cephalic appendages, acting both 

 as jaws and legs. 



No thoracic segments. 



Trelobita. 



Body large and massive, consisting of a head, form- 

 ing a broad cephalic shield ; thorax, and abdomen dis- 

 tinctly trilobate. 



Frontal doublure well marked. 



Compoimd eyes many facetted; simple eyes wanting. 



Hypostoma large, squarish. 



? 



Thoracic segments developed at, and after, the first 

 moult of larva. 



Abdomen with segments partially free or coalesced, 

 ending occasionally (Encrinurus, Dalmania) in a spine, 

 or flattened telson (Lichas). 



? 



Larva when hatched, in some genera differing from 

 parent in wanting thoracic segments. 



Abdomen with segments free or coalesced, forming 

 a broad shield ending in a spatulate or spine-like 

 telson. 



Broad lamellate, respiratory, abdominal feet. 



Larva, when hatched, like the parent, consisting of 

 head and abdomen only. 



It appears from the above review that the only fundamental characters separating the two 

 groups, is the addition in the TrUobites, at or after the first moult, of thoracic segments. 

 We should in this connection bear in mind that in the Phyllopoda the number of body seg- 

 ments is constantly varying in different genera, as it is among the Trilobites and Merosto- 

 mata themselves. The close relationship of the two groups is farther shown by the recent 

 discovery of some remarkable problematical forms by which the Merostomata and Trilobites 

 are apparently linked together. These forms are Hemiaspis (Woodward ),^ Pseudoniscus 

 (Nieszkowski),^ Exapinurus (Niesz.), and Bunodes (Eichwald). 



iThe late Mr. Salter (Quart. Journ. Geol. Soc, vol. xix, 

 p. 87) has been the first to recognize the close relationship 

 of the fossil genera enumerated above, with the modern 

 Nebalia. It had also occurred to the writer previous to 

 meeting with Mr. Salter's article, that Ceratiocaris was a 

 gigantic Ncbaliad. He says, " I have had occasion to study 

 a large series of Silurian Crustacea, which have, in their 

 turn, furnished the data whereby to connect this old Phyl- 

 lopod (speaking of Peltocaris) with the modern Nebalia and 



Apus." We are not convinced that there is any real genetic 

 connection between Apus and these fossil Nebahads,' but 

 Mr. Salter's remarks are highly suggestive. 



2 On some Points in the Structure of the Xiphosura, hav- 

 ing reference to their relationship with the Eurypteridoe. 

 By H. Woodward. (Quart. Journ. Geol. Soc, Feb., 1867.) 



' Zusiitze zur Monographic der Trilobitcn der Ostsee- 

 provinzen, nebst der Beschreibung einiger neuen obersilu- 

 rischen Crustaceen. Dorpat, 1860. 



