ON THE CARBONIFEEOUS XIPHOSUROUS FAUNA. 155 



II. — Affinities of the Eurypterida to the Xiphosurn {Limulidm) and the formation of the order Mere- 



stomata as at present received. 



In 1825 Dr. J. E. De Kay described and fi};iired the fii-st (iiu American) species of Eurypterus 

 known (E. reniipes), and referred it to the class Crustacea and to the order Branchiopoda. 

 In ISi^ L. Agassiz remarked of Pterygotus : 



I am rather iuclinetl to believe that this singular auimal will become the type of a family intermediate between 

 the TVilobites and the Entomosiracans in which perhaps, the Eurypteri and the EidothecB w\]\ some day be included. 



We have given on pp. 177, 17S'of our essay on " The Development of Limulns" (1872) a history 

 of the views of James Hall, Salter, and others, especially the first-named, who proved that the 

 Eurypterida belonged to the same order as Limulus. 



In 1866 in his elaborate " Monograph of the British fossil Crustacea, belonging to the order 

 Mesostomata," Dr. H. Woodward formally united the Eurypterida in the same order with Limu- 

 lus, remarking : 



Having long been eonvinced of the propriety of expressing in some suitable manner the correctness of the conclu- 

 sions of Professors Agassiz and James Hall as to the close affinity existing between the Eurypterida and thw Xijyhosura, 

 and being fully persuaded at the same time that they naturally form two distinct although closely related groups, I 

 have ventured to unite them in the Order Merostomata— a name proposed by Dr. J. D. Dana for the recent king- 

 crabs only, retaining at the same time the names Eurijpterida and Xiphosiira as suborders. 



In 1872 we adopted this classification, which seems eminentlj' natural, and has since been 

 adopted by a number of leading zoologists. 



In 1868 Clans (Gruudziige der Zoologie) characterized the order Poecilopoda, but in the third 

 edition of this work (1876) the Poecilopoda (restricted to Limulus), though placed between the 

 fourth order, Phyllopoda, and fifth order, Arthrostaca, in the Crustacea, and at the end of the Phyl- 

 lopoda, are associated with the Trilobita in a special group to which no special rank is assigned. 



III. — Transfer of the Merostomata {tcith the Trilobita) to an independent class. 



In 1869 Huxley stated in the "Academy" (November 13) : 



The Xiphosura have such close morphological relations with the Arachnids, and especially with the oldest known 

 Arachnidan, Scorpio, that I cannot doubt the existence of a genetic connection between the two groups. 



In 1871 Prof. E. Van Beneden (Comptes Rendus de la Soc. Ent. Belgique, October 14, 1871 ; 

 Annals and Mag. Nat. Hist., .January, 1872) remarked : 



The Limuli are not Crustacea ; they have nothing in common with the Phyllopoda, and their embryonic devel- 

 opment presents the greatest analogy with th.at of the scorpions and other Arachnida, from which they cannot be 

 separated. ♦ » • The Trilobites, as well as the Eurypterida and the Pcecilopoda, must be separated from the class 

 Crustacea, and form with the Scorpionida and the other Arachnida a distinct branch, the origin of which has still to 

 be ascertained. 



In 1872 A. Milne Edwards (Annales des Sc. Nat.) published his important researches on the 

 internal auntomy of Limulus, which showed that Limulus essentially differs from the Crustacea. 

 In the same year we attempted to show the close aflauities of Trilobites to Limulus. 



In 1876, according to Claus's own statement (Annals and Mag., July, 1886, p. 56), referring to 

 his change of views as to the position of Limulus, he remarks : 



Even in the work entitled " Uutersuchungen uber die gonealogi.sche Grundlage des Crustaceensystems" (Vienna, 

 1876) I adhered to the views of those who, like Straus-Diirckheim, regard Limulus and Branchiate Gigantostraca as 

 allied to the air-breathing Aracbnoidea, and the latter as baving proceeded from the former, although, having regard 

 to the possibility of a still uudemonstrated Nauplius stage, I considered it probable that the common origin of the 

 true Crustacea was rather after than before the Nauplius period of the Stem-Crustacean. In the case of Limulus and 

 the Scorpions I also asserted the homology both of the six pairs of limbs of the cepbalotborax, and, with reference to the 

 developmental history, of the six pairs of limbs of the pneabdomen, of which the second pair repre.seuts the comb-like 

 organ of the Scorpions, while the following four pairs immediately undergo retrogression (p. 110). In the " Gruud- 

 ziige der Zoologie" of the year 1880 I went so much further as to divide the Branchiata, or Crustacea, sensu laiiori, 

 into Encrustacea (with the Entomostraca and Malacostraca) and Gigantostraca (with no certain traces of the Nauplius 

 stage), and accordingly I affirmed expressly of the Tracheata that in opposition to the more ancient Branchiata they "ictre 

 not referable to a unitary origin, sinct the Arachnoidea, which are derivable from the Gigantostraca, stand opposite to the 

 Myriapoda and Insecta, which are united by a closer affinity " (p. 515). 



