ON THE CARBONIFEROUS XIPHOSUROUS FAUNA. 155 
Il.— Affinities of the Eurypterida to the Xiphosura (Limulide) and the formation of the order Mero- 
stomata as at present received. 
Tn 1825 Dr. J. E. De Kay described and figured the first (an American) species of Eurypterus 
known (£. remipes), and referred it to the class Crustacea and to the order Branchiopoda. 
In 1844 L. Agassiz remarked of Pterygotus: 
I am rather inclined to believe that this singular animal will become the type of a family intermediate between 
the Trilobites and the Entomostracans in which perhaps, the Euryptert and the Hidothee will some day be included. 
We have given on pp. 177, 178 of our essay on “ The Development of Limulus” (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. 
Tn 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 Zurypterida and the Xiphosura, 
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 Lurypterida and Xiphosura as suborders. 
In 1872 we adopted this classification, which seems eminently natural, and has since been 
adopted by a number of leading zoologists. 
In 1868 Claus (Grundziige der Zoologie) characterized the order Pecilopoda, but in the third 
edition of this work (1876) the Pecilopoda (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. 
Il1.— Transfer of the Merostomata (with the Trilobita) to an independent class. 
In 1869 Huxley stated in the “ Academy” (November 18) : 
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 that of the scorpions and other Arachnida, from which they cannot be 
separated. * * * The Trilobites, as well as the Eurypterida and the Pecilopoda, 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 Se. Nat.) published his important researches on the 
internal anatomy of Limulus, which showed that Limulus essentially differs from the Crustacea. 
In the same year we attempted to show the close affinities 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 “‘ Untersuchungen iiber die genealogische 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 Arachnoidea, and the latter as having proceeded from the former, although, having regard 
to the possibility of a still undemonstrated 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 cephalothorax, and, with reference to the 
developmental history, of the six pairs of limbs of the preabdomen, of which the second pair represents the comb-like 
organ of the Scorpions, while the following four pairs immediately undergo retrogression (p. 110). In the ‘‘ Grund- 
ziige der Zoologie” of the year 1880 I went so much further as to divide the Branchiata, or Crustacea, sensu latiori, 
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 ‘were 
not referable to a wnitary origin, since the Arachnoidea, which are derivable from the Gigantostraca, stand opposite to the 
Myriapoda and Insecta, which are wnited by a closer affinity” (p. 519). 
