18 THORELL & LINDSTRÖM, SILURIAN SCORPION FROM GOTLAND. 



pioidce '[_-nidce]. By far the most iraportant of these deviations occurs in the form of 

 tlie legs, a form so peculiar and so decidedly unlike what has hitherto been met with 

 in this group of animals, that one might have felt uncertain whether Palaaophonus 

 really is an arachnid, had not its perfect conformity with the scorpions in all other 

 essential points, and especially in the presence of stigmata (spiracula) on the ventral 

 plates, excluded every doubt concerning its systematic position. Whilst in all other 

 scorpions the legs, especially the hinder ones, are long and slender; the tibial and 

 the femoral joints in particular much longer than broad, the three joints of the tarsi 

 of nearly the same thickness, and the third tarsal joint eylindrical or somewhat club- 

 shaped, blunt, and armed with two viovable claws; in Pateophonus the legs are short 

 and dumpy, the tibial and especially the femoral joints are but slightly, if at all, lon- 

 ger than broad, the joints of the tarsi gradually taper towards the apex, and their third 

 joint is long and conical, sharply pointed and either without claws or perhaps provided 

 v-ith a single minute and immovable claw. The legs of Palaiophonus belong to a type 

 which, as far as we know, had disappeared already in the scorpions of the Carbonife- 

 rous period: it is never met with in recent scorpions, not even in their embryos^), 

 but it may be observed in the early developmental stages of several other widely dif- 

 ferent gi'oups of Arthropods, for instance in the embryo of Limulus; in the embryo of 

 some insects as for example Phryganea^), in the Eurypteridaj^), and iu several Decapods 

 as Hippa, in Palseocampa, Campodea, etc. 



Another very important feature, by which Palajophonus (at least the Scotch 

 species, according to Peach) differs from both recent and Carboniferous scorpions, 

 may be observed in the arrangement of the posterior coxas: for whilst in Pala^opho- 

 nus, only the coxaä of the fourth pair abut against the sternal plate, which is bounded 

 along its anterior margin by the coxa? of the third pair; in other scorpions the coxse 

 of both these pairs abut against the sternum, which is bounded in front by the coxse 

 of the second pair. 



Palseophonus is further distinguished from recent scorpions by the deep transverse 

 furrow which extends across the posterior part of the cephalothorax, parallel to the 

 articulations between the dorsal plates of the abdomen, and marks the limit between 

 the two posterior of the four (thoracic) segments, which, in scorpions and most other 

 arachnids, have coalesced with one another and the head, and which bear each a pair 

 of walking limbs. In recent scorpions, this fi^rrow is less distinct, being only repre- 

 sented by a more or less obliquely transverse groove on both sides of the cephalothorax. 

 In this feature Paleeophonus seems more to agree with some of the scorpions of the 

 Carboniferous period. In a few of the species of Uoscorjnus of Peach, as in E. infla- 



') According to Eathke and Metschnikoff the embryos of the Scorpions have thiok, bluntly ending legs, 

 but Leon Dufode in his »Histoire anatomique et physiologique des Scorpions» has given a figure, of »Scor- 

 pio européns» pl. 4 fig. 41, showing legs with pointed extremities, nearly resembling those of Palaeophonus. 



^) Zaddach, Untersuchungen iiber dia Entwiokelung und den Bau der Gliederthiere. l:s Heft, Die Entwicke- 

 lung des Phryganiden-Eies. Berlin 1854. See especially Pl. 4, figs. 42, 53, 56. 



^) See for instance the median pairs of extremities in the figure of Eurypterus obesus WooDw., which Wood- 

 WARD has given in his Monograph of the British fossil Crustacea of the Order Merostomata, Part IV, p. 

 160 (The Paleeontogr. Society, 1872). 



