150 PACKARD—CLASSIFICATION OF ARTHROPODA. [April 3, 
of Aglaspis eatoni with that of Dalmanites (Figs. 1414 and 1331 
of Zittell-Eastman’s Paleontology), is there not such a close 
resemblance in the shape of the head (or cephalothorax) and of 
the trunk-segments as to suggest a close alliance, even though mem- 
bers of two distinct classes? To answer this question by saying 
that this is a case of convergence, the objector might be referred to 
the other Synxiphosura, which also suggest a common origin of the 
two classes from a protaspis ancestor. It has been suggested that 
some of the Cyclidz are larval Eurypterida or Limuloids ; if this 
should prove to be the case (of which we are by no means sure, as 
no Cyclidz have yet been found below the Carboniferous), we should 
have an additional argument for the common origin of the two 
classes, for the Cyclidz somewhat resemble the larval trilobites. 
Relation between the Merostomata and Arachnida.—While we 
have from the first maintained that the Merostomes should not 
actually be included among the Arachnida—v. ¢., that Limulus is 
not a genuine Arachnidan, as claimed by Van Beneden, Lankester 
and later authors—from the evidence we now have as to the mode 
of origin of the book-lungs and the morphogeny of the appendages 
in general, and especially the interesting and remarkable discovery 
by Mr. Pocock,! that the Silurian so-called scorpions are probably 
marine links between scorpions and Eurypterida, whatever objec- 
tions I have formerly expressed to their Arachnidan affinities are 
now overcome, and it seems plain that the scorpion is a direct 
descendant of some Eurypteridan. Pocock’s fortunate discovery 
in the Silurian scorpion (Paleophonus hunteri) of the inner branch 
of a two-jointed appendage, which appears to be the homologue of a 
recent scorpion’s ‘‘pecten,’’ should it be confirmed by the dis- 
covery of additional examples ; as well as the thickness of the head- 
appendages, the last four pairs of which end in a single point, not 
in claws, as in modern scorpions—these discoveries appear to give 
the clue to the line of descent of Arachnida from some Merostome, 
one would say from some Eurypterus-like form, though, from other 
features observed by him, Mr. Pocock takes the view that ‘‘ Palzo- 
phonus occupies an intermediate position between Limulus and the 
Eurypterida on the one hand and recent scorpions on the other, 
standing, if anything, rather nearer to the former than to the 
latter.’’ 
1« The Scottish Silurian Scorpion,” Quart. Fourn. Micr. Sc., Vol. 44, 0. S., 
1902. 
