Page:Pocock, The Scottish Silurian Scorpion.pdf/7

 attached to its apparent absence, in view of the chances against the preservation of an organ so delicate.

Nor was I able to detect a sign of the presence on the fifth segment of any of the legs of that spur so clearly shown on the first, second, and third pairs in the Gotland fossil, and described and figured by Thorell (see cut, p. 296). The interest invested in this spur depends upon the probability of its direct homology with the so-called "tibial spur" found upon the arthrodial membrane at the distal end of the fifth segment in some recent Buthoid scorpions. Certain genera of this family (e.g.,  ) possess it upon the third and fourth legs, one alone  retaining it only on the fourth leg. Assuming that the spurs in the genera just mentioned are homologous to those found in the Swedish, their presence upon the third and fourth, or upon the fourth leg in the former, and upon the first, second, and third legs in the latter, suggests that scorpions primitively possessed them upon all four legs. In that case the absence of the spur from the fourth leg in the type of  may be a natural characteristic of the species, or may be due to a mere accident of preservation. The same may be said of the apparent total absence of this spur from the legs of the Scotch specimen.

There is, however, a still deeper interest attached to this spur, on account of its apparent presence upon the fourth leg (sixth prosomatic appendage) of. The first and second appendages of this animal agree in structure and in the number of segments with those of scorpions, the former consisting of three and the latter of six segments. But the third, fourth, and fifth appendages of  also consist apparently of six segments, resembling in all particulars those of the second pair. In the scorpions, on the contrary, these appendages, as well as the sixth pair, consist of seven segments, the distal being furnished with a pair of moveable claws. Careful examination of these appendages in, however, shows that the fourth segment is encircled in its basal