THE SCOTTISH SILURIAN SCORPION. 297 



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. Buthus, Lychas) posssess it upon the 

 third and fourth legs, one alone (Baby cur us) retaining it 

 only on the fourth leg. Assuming that the spurs in the 

 genera just mentioned are homologous to those found in the 

 Swedish Palasop bonus, 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 leo-s. 

 In that case the absence of the spur from the fourth leo- 

 in the type of Pala^ophonus nuncius may be a natural 

 characteristic of the species, or may be due to a mere 

 accident of pi'eservation. 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 Limulus. 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 Limulus 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 seg- 

 ments, the distal being furnished with a pair of moveable claws. 

 Careful examination of these appendages in Limulus, how- 

 ever, shows that the fourth segment is encircled in its basal 



