THE SCOTTISH SILURIAN SCORPION. 301 



A very different state of things appears to obtain in 

 Palaeoplionus. No trace of a sterno-coxal process is dis- 

 coverable upon the first leg. On tlie second, liowever, 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 canuot, 

 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., PI. 19) does not stand out as a sharply 

 defined plate with clean-cut edges, but 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 

 pi-esumably to the similarly shaped sternal depression 

 in Chserilus, 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 Limulus or one of the Eurypterida. The relations of 

 the sternum to the coxEe and the coxee to each other in the 

 scorpions have already been described. Those of Limulus 

 and the Eurypterida may be stated in a very few words. In 

 the latter the basal segments of all the appendages, ex- 



