ON THE CARBONIFEROUS XIPHOSUROUS FAUNA. 151 



The individual development of Limulus an epitome of that of the Xiphosura. 



It is interesting to observe a clearly marked exemplification of the parallelism between the 

 embryonic or ontogenetic development of Limnlus and the geological succession as well as evolu- 

 tion of the suborder of which it is a type. We have already compared the orbiculo-hemis- 

 pherical form of Cyclus with that of Limulus in the early stages of its embryonic life. The par- 

 allelism is striking. Cyclus may therefore be called an embryonic form. Again, in Prestwichia 

 there is a close resemblance to Limnlus shortly before it leaves the egg, in what we have called 

 the trilobitic stage, a stage antecedent to the true larval stage, in which the abdominal segments 

 become consolidated. Prestwichia may then be properly designated as a larval form, while Cyclus 

 was an embryonic form. The latter genus embraces eleven species (ten in Europe), which exist in beds 

 containing the species of Belinuridae. One cannot regard it as a retrograde form however, but as an 

 embryonic Xiphosuran, whose development became accelerated, adapting it for active adult life. 

 WLile the specimens of Cyclus have not yet shown the presence of compound lateral eyes, it is 

 not impossible that the animal was provided with a pair of median simple eyes. This indicates 

 that these were the primitive visual organs, and that the compound lateral eyes of the Belinuridse 

 and Limulidaj were secondary acquisitions, and that their simple eyes ai'e legacies left by their 

 Cyclus-like ancestors. 



Cyclus, and perhaps Dipeltis, appear to represent Agnostus among Trilobites, and the sim- 

 ilarity between all these simple types indicates a community of descent. 



The Suborder SYNZIPHOSURA. 



In the Upper Silurian beds of Europe have been revealed a number of exceedingly interesting 

 forms, which appear to be Merostomata, but not true Xiphosura. They serve, on the one hand, 

 to connect the Xiphosura with the Eurypterida, and also strongly suggest the community of origin 

 of the Merostomata and Trilobita. They have been associated by Dr. Woodward with the Euryp- 

 terida,* but it seems to us, in the light of our present knowledge of the latter suborder and of the 

 Xiphosura, that they are types of a third group or suborder. Perhaps the more aberrant form is 

 Buuodes of Eichwald. All the genera have a caudal spine or telsou. They are, besides Bunodes, 

 Hemiaspis Woodward, Pseudoniscus Nieszkowski, Exapinurus Nieszk., and perhaps JSTeolimulus 

 Woodward belongs with them, though the last form connects the Xiphosura and Synziphosura. 

 They possess nearly as high an antiquity as the Eurypterida, but did not persist so long, as none 

 have been discovered in the Devonian or Carboniferous rocks ; hence we would infer that they were 

 the forerunners of the Xiphosura rather than actual members of the group. In a word, the mero- 

 stomatous ordinal tree divided into three main branches — /. e., the Eurypterida; the forms under 

 consideration, which may be designated as the Synziphosura ; and the genuine Xiphosura. In 

 the Synziphosura the head forms a solid plate, with a slightly marked glabella or median lobe. 

 Compound eyes are present in Pseudoniscus, and in Exapinurus the head is produced laterally 

 into large genal spines. All have free uromeres or abdominal segments, and in all except Bunodes, 

 in which the pleurum is shaped and marked as in Trilobites, the uromeres possess lateral projec- 

 tions or spines. None of them show traces of limbs or of simple eyes, and all are of moderate size. 



The Synziphosura may be divided into three families, which may be diagnosed as follows 

 (these groups appear to be, on the whole, equivalent in rank to the families of Trilobites) : 



1. Head rounded; no geual spiue; abdomen divided into a "thorax," consisting of six trilobite-like segments, 

 with diagonal pleural lines; "abdomen" of four segments, besides the large telson (Bunodes and Exapinurus). 



Bunodidw Packard. 



2. Head one-half broad as long, with several genal spines; abdomen triangular, with nine segments and a short 

 telson (Hemiaspis). Bemiaspidce Zittel (restricted). 



3. Body oval ; head short ; large compound eyes ; nine abdominai segments besides a short telson (Pseudoniscus). 



Pseudoniscida; Packard. 



4. Head-shield short and broad ; abdomen very broad, of nine segments besides the telson (Neolimulus). 



NeolimulidcB Packard. 



' Quart. Journ. Geol. Soc, Feb., 1667. 



