634 REPORT— 1902. 



becomes necessary to compare the Scorpion with the other Arachnida, Limulus 

 with the Eurypterida, in order the better to determine the position of each in its 

 respective series, by the application of this rule. 



As to the number of segments present, variation is a matter of small concern, 

 in consideration of the mode of origin of segmentation and the wide numerical 

 range — from seven in the Ostracods to more than sixty in Apus — the segments of 

 the crustacean class present. 



On the arachnidan side, in the Solifugae but the third and fourth segments are 

 fused ; the remaining four of the prosomal series with the ten which remain are 

 free.^'^ In Kcenenia four of the prosomal segments alone unite ; the fifth and sixth 

 with the rest are free."- And when we pass to the Limuloids and the descending 

 series of their allies, we find it distinctive of the Eurypterida that all the opistho- 

 somal segments are free. If we can trust these comparisons, we must conclude 

 that the Eurypterida of the past, in respect to their segmentation, simplify the 

 Limuloid type, on lines similar to that on which the Solifugae and Kcenenia 

 simplify the Higher Arachnid and Scorpionid type, and that therefore if the degree 

 of antero -posterior fusion of segments has the significance attached to it, Limulus 

 and Scorpio must each stand at the summit of its respective series. If this be 

 admitted, it has next to be asked if, in comparing them, we may not be comparing 

 culminating types, which might well be isomorphic. 



The scorpions are known fossil by two genera, Palaophonus and Proscorjmts, 

 from the Silurian of Gotland and Lanarkshire, the Pentland Hills, and New York 

 State ; "^ while recent research, in the discovery of the genus Strabop.i, has traced the 

 Eurypterida back to the Cambrian, leaving the scorpions far behind."^ One striking 

 feature of the limbs of thePalseozoicEurypterids is their constantly recurring short- 

 ness and uniformly segmented character, long known in Slitnoin'a, Ewypterus, and 

 Pterygotiis, retained with development of spines in some genera, and for three of 

 the five known appendages of the recently described eurypterid giant Istylonurus 

 lacoanusP" The minimum length observed for these appendages is that of the 

 Silurian species Eurypterusjisclieri, discovered by Holm in Russia in 1898.^"^ This 

 creature is one of the few eurypterids in which all the appendages are preserved, 

 and it is the more strange therefore that the advocates of the arachnid theory 

 should ignore it in their most recent account. Allowing for the specialisation of 

 its sixth prosomal appendage for swimming, the fifth is but little elongated, the 

 second, third, and fourth are each in total length less, by far, than the transAerse 

 diameter of the prosoma, and uniformly segmented, giving the appearance of short 

 antennse. They seem to be seven-jointed, and are just such appendages as exist in 

 the simpler crustacean and tracheate forms ; and in the fact that their structural 

 simplicity is correlated with the independence of the whole series of opisthosomal 

 segments they lend support to the argument for isomorphism. 



With this conclusion, we turn once more to the Scorpions, if perchance something 

 akin to it may not be in them forthcoming. The Silurian genus Palceophonus, 

 especially as represented by the Gotland specimen, reveals the one character desired. 

 Its body does not appear to be in any marked degree simpler than that of the living 

 forms ; but on turning to its limbs, we find the four posterior pairs, in length much 

 shorter than those of any living species, all but uniformly segmented."' In this 

 they approximate towards the condition of the limbs of the Eurypterida just dis- 

 missed, and their condition is such that had they been found fossil in the 

 isolated state they would have been described as the limbs of a Myriapod, and not 

 of a scorpion at all. Indeed, their very details are what is required, since in the 

 possession of a single terminal claw they differ from the limbs of the recent 

 scorpions as do those of the Chilopoda from the hexapods. 



With this the scorpionid type is carried back, with a structural simplification 

 indicative of a parallelism with the other arthropod groups ; and while the facts 

 do not prove the total independence of the scorpionid and limuloid series, they 

 bring the latter into closer harmony with the Eurypterida of the past. They 

 prove that the Silurian Scorpions simplify the existing Scorpionid type, on precisely 

 the lines on which the Eurypterida simplify the Limuloid ; and they do so in a 

 manner which suggests that a distinction between the Cnistacea vera and the 



