THE EURYPTERIDA OF NEW YORK I41I 
dealt with by Laurie. Lankester had already indicated the close homol- 
ogies existing in the number of body segments of the cephalothoracic 
appendages [see especially op. cit. tabular statements p. 536].!| From these 
it would follow that Limulus on the one hand, Scorpio and the euryp- 
terids on the other, separated before the consolidation of the body segments 
observable in Limulus; but Laurie states that the second abdominal seg- 
ment in Scorpio is well developed and shows no sign of ever having been 
suppressed by the genital operculum as in the eurypterids. From this 
argument only it might seem that the scorpions came off the eurypterid 
stem before the great development of the genital operculum. Laurie 
considers this development of the genital operculum at the expense of 
the second free segment as a point of considerable morphological import- 
ance and has therefore [op. cit. 526, see also Recent additions, etc. p. 127] 
considered it probable that the Pedipalpi (Thelyphonus) are more nearly 
allied to the eurypterids than are the scorpions, for in the former a similar 
suppression of the second ventral segment has taken place in favor of the 
genital plate. The four scorpions known from the Siluric all exhibit char- 
‘A brief summary of the resemblance of the eurypterids with the scorpion has 
been lately given by Woods [1909, p. 283]. It reads as follows: 
The eurypterids present a striking resemblance to scorpions. In both groups the 
segments in the three regions of the body are the same in number, and the appendages 
of the prosoma also agree in number and position. The preoral appendages are chelate 
in both, but the second pair of appendages are chelate in the scorpions only. In 
eurypterids the coxae of the five pairs of legs are toothed and meet in the middle line, 
but in the scorpions the coxae of the last two pairs do not meet; this difference, how- 
ever, appears to be bridged over in the earliest known scorpion—Palaeophonus, from 
the Silurian rocks. The eurypterids are distinguished from the scorpions by the much 
greater development of the last pair of legs. The large metastoma of the former is 
homologous with the sternum of the scorpion. The genital operculum is much smaller 
in scorpions than in eurypterids, and in this respect the latter agree with Thelyphonus 
(one of the Pedipalpi) more than with the scorpions. The pectines are absent in the 
eurypterids except in Glyptoscorpius. Instead of the lung books of the scorpions the 
eurypterids possess branchial lamellae on the platelike appendages; but this difference 
between the two groups appears to be bridged over by Palaeophonus, which was 
marine, and may have possessed branchial lamellae since stigmata seem to be absent. 
