Page:Pocock, The Scottish Silurian Scorpion.pdf/11

 A very different state of things appears to obtain in. No trace of a sterno-coxal process is discoverable upon the first leg. On the second, however, a small one seems to be present. This lies transversely, and meets its fellow of the opposite side in the middle line. On the third leg a process similar in its form and relations is also indicated, and the segment that bears it, instead of abutting against the sternum, is mesially in contact with its fellow. The probability of the correctness of this conclusion is enhanced by its tallying with Peach's opinion. I cannot, however, quite agree with this author in believing that the legs of the fourth pair are basally separated by the sternum as in recent scorpions. On the left side of the specimen, where the leg is well preserved, the segments seem to be traceable right up to the middle line, the basal segment being sharply defined. On the right side, however, this is not so clearly indicated, on account of a displacement which has resulted in the overlap of the proximal end of the fourth leg by that of the third.

The sternum (st., Pl. 19) does not stand out as a sharply defined plate with clean-cut edges, bnt is merely represented by the subpentagonal area that lies between and behind the two proximal segments of the fourth leg of the left side, and those of the third and fourth legs of the right side. It shows a faint central circular depression answering presumably to the similarly shaped sternal depression in, and to the median groove in other recent scorpions.

The above-described arrangement of the skeletal pieces, forming the ventral surface of the prosoma, offers many points of morphological importance in view of the differences that obtain in this particular between the recent scorpions and  or one of the Eurypterida. The relations of the sternum to the coxæ and the coxæ to each other in the scorpions have already been described. Those of  and the Eurypterida may be stated in a very few words. In the latter the basal segments of all the appendages,