126 NATURAL SCIENCE. Aug.. 



in Stylonums, where more than half of the under sufacee is covered in 

 this way. 



The ventral surface of the free segments of the body is much 

 less well-known than that of the cephalothorax. The first two seg- 

 ments are covered by the genital operculum, which consists of a pair 

 of plates meeting in the middle line and having a median lobe attached 

 to them. This median lobe varies in form in the same species 

 according to the sex, and may be considered as a copulatory organ 

 in the male and an ovipositor in the female. In some species, e.g., 

 Slimonia, one form of this organ shows distinct signs of having been 

 eversible. Underlying this genital operculum are a number of leaf- 

 like structures, almost certainly branchial in function, which appear 

 to have been attached to the body-wall, not to the operculum, and 

 which, therefore, probably represent the reduced appendages of the 

 second free segment. This segment has lost^its ventral hard skeleton, 

 and the appendage has become reduced owing to being covered by 

 the genital operculum. 



The appendages of the segments behind these first two have only 

 recently beenj made out. Schmidt (4) describes them in Euvypterus 

 fischeri as consisting of a series of four pairs of plates, each pair 

 united in the middle line, and covering the whole ventral surface of 

 the segment to which it belongs. The ventral surface of the body is, 

 according to his description, entirely devoid of sclerites in these 

 segments, and the plate-like appendages bear branchiae on their 

 posterior surfaces. He describes, though with less detail, a similar 

 arrangement in Pterygotus ossiliensis. In Slimonia, on the other hand, 

 the abdominal appendages have been described (6) as consisting also 

 of four pairs of plates, which, however, with the exception of the 

 first pair, do not meet in the middle line, but are successively smaller 

 from before backwards. The ventral surface of the segments has, 

 further, well-developed sclerites extending across the whole width of 

 the body. In both cases the chief point is established that the 

 Eurypteridse had plate-like abdominal appendages bearing branchiae 

 on their posterior surfaces, and, in a general way, comparable to those 

 of Limulus. 



Fragments of some very interesting forms have been described 

 from the Lower Carboniferous by Peach (7) under the name of Glypto- 

 scorpius. These fragments appear to have belonged to a large animal 

 (over I foot in length), the surface of whose body was covered with 

 highly- developed scale markings. The limbs ended in a double claw 

 similar to that of the Scorpions, and the animal was further provided 

 with a pair of appendages closely resembling in structure the pectines 

 of Scorpions. This form was probably aquatic in its habits, and 

 represents not the direct ancestors of Scorpions, for they had appeared 

 long before, but a collateral branch. True Eurypterids existed simul- 

 taneously with this form, though possibly many so-called Eurypterid 

 fragments from the Carboniferous belong really to Glyptoscorpius. 



