THE SCOTTISH SILURIAN SCORPION. 307 



by means of the appendages of the second mesosoixiatic 

 somite, although it must be admitted they appear too small to 

 have performed this office for the whole organism without 

 help from other organs. It is possible that there were such 

 organs in the form of small appendages bearing branchial 

 lamellge attached to the mesosomatic sterna. But if so, no 

 definite trace of such has been preserved. Or, indeed, it is 

 possible that the ventral plates, above regarded as meso- 

 somatic sternites, may have been broadly laminate mesosomatic 

 appendages, closely pressed down against the ventral surface 

 of this region, and bearing branchial lamellae on their pos- 

 terior surfaces. This suggestion gains some support from the 

 fact that the laminate mesosomatic appendages of the 

 Eurypterida are generally indistinguishable from sternal 

 plates. 



Metasoraa. — This region of the body in the Scotch speci- 

 men closely resembles that of the Swedit<li specimen, the same 

 surface, namely the ventral, being in each case uppermost. 

 Peach, however, states that the dorsal surface of the posterior 

 caudal segments is in part exposed. According to my inter- 

 pretation, on the contrary, in all the segments it is the area 

 lying between the inferior lateral keel on the left side {inf. 

 led. and sujo. lat., PI. 19) and the superior lateral keel on the 

 right that is exposed. Both of these keels are granular. As 

 in most recent scorpions, a pair of median keels [inf. med., 

 PI. 19) lie along the lower surface of the tail, between the 

 inferior lateral keels on the first four segments of the tail. 

 Keels corresponding to these four inferior medians and 

 inferior laterals are traceable upon the first metasomatic 

 sternite, and also, I think, upon the sixth (fifth caudal 

 segment). This last fact, if true, is of some interest, inas- 

 much as it shows a more primitive arrangement than is found 

 in recent scorpions, where the two median keels have invariably 

 coalesced into one. The inferior median keels on the pos- 

 terior caudal segments appear to be smooth. In the Swedish 

 specimen they are granular. The lower side of the vesicle is 

 granular in both, but the aculeus in the Scotch example is 



