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THE EURYPTERIDA OF NEW YORK

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dealt with by Laurie. Lankester had already indicated the close homologies existing in the number of body segments of the cephalothoracic appendages [see especially op. cit. tabular statements p. 536].^[1] From these it would follow that Limulus on the one hand, Scorpio and the eurypterids on the other, separated before the consolidation of the body segments observable in Limulus; but Laurie states that the second abdominal segment 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 importance 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, however, 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.

[1] 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, however, 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.

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