DEVELOPMENT OF LQIULUS POLYPHEMUS. 191 



Of these, Pseudoniscus and Exapinurus seemed nearest allied to Bellinurus, though Mr. 

 Woodward refers them without hesitation to the Eurypterida, while they seem to me to form 

 types of a division intermediate between the Limulidoe and the Eurypteridse. As to the 

 genera Hemiaspis and Bunodes, they present a remarkable resemblance, whether of analogy 

 or affinity I would hardly venture to say, with the Trilobites, and even possibly indicate a 

 group intermediate between the Eurypteridte and the Trilobites. In both genera the body 

 is distinctly trilobed, thus (Hemiaspis especially) resembling Bellinurus, (the body of the 

 true Eurypteridse being smooth). Bunodes, as represented by Nieszkowski, bears a remark- 

 able resemblance to Asaphus and Blsenus, in the broad, flat head (though no eyes are 

 present), the five segments between the head and abdomen, and in the form of the latter. 



In their most essential characters, i.e., the broad, cejAalic shield, the form of the frontal 

 doublure, the large metastoma (of the Eurypteridae) the compound eyes, and in a less 

 degree the form of the abdomen in some genera, the Merostomata are more closely approxi- 

 mated to the Trilobites than to the Phyllopoda. Add to this the almost identical form of 

 the animals when hatched, and the probability that the mode of development of the 

 embryo of the Trilobites accorded in its main features with that of Limulus, the type of an 

 important group of Merostomata, I feel as if we were almost justified in placing the two 

 groups as subdivisions of an order of the subclass Branchiopoda, and perhaps equivalent to 

 (as much so as natural groups are) the Phyllopoda and Cladocera. 



It may seem to naturalists as if it were forcing the Trilobites and Merostomata into an 

 unnatural aUiance with the Phyllopoda, but they agree (at least the Merostomata) in having 

 broad, lamellate giU-bearing feet and a more or less developed cephalic shield bearing 

 compound eyes ; the Eurypteridse with their small head parallelizing the Branchipodidae 

 in their want of a broad cephalic shield. Then the two groups may be represented thus : 



Cladocera and Phyllopoda. Merostomata and Trilobites. 



Geological Succession and probable Ancestry of the Branchiopoda. The following 

 table may serve to give a rude idea of the relations of the principal groups of the Ento- 

 mostraca, and their appearance in geological history, so far as the extremely scanty data 

 we possess will allow, while the diagram may also serve as a genealogical tree, showing the 

 probable origin of the main divisions of the lower Crustacea. 



As is well known, the Trilobites are met with in comparative abundance in the lowest 

 fossiliferous beds of the Silurian period, and they are the most ancient of Crustaceans, so 

 far as their remains give evidence. The genera Conocephalites, Dicellocepbalus, Paradoxides 

 and Agnostus, besides other forms, appear in the Potsdam sandstone or equivalent primor- 

 dial rocks of this and other countries. The type disappeared during the Carboniferous 

 period, the genera Phillipsia (one species is Permian), Griffithides and Brachymetopus, being 

 the sole representatives of the type which prevailed so extensively during the Silurian. 



