Page:Pocock, The Scottish Silurian Scorpion.pdf/21

 structure of the appendage of the second somite of the mesosoma.

If the above-given interpretation of the arrangement of the parts constituting the ventral side of the prosoma is correct—and I do not think it is likely to be very far wrong—Palæophonus occupies an intermediate position between Limulus and the Eurypterida on the one hand, and recent scorpions on the other, standing, if anything, rather nearer to the former than to the latter.

The same may be said of the structure of the second mesosomatic appendage, which with its outer and inner branch is like the corresponding appendage in ; while the outer branch itself, although in general form and size resembling the pecten of a scorpion, offers some interesting structural features in which it differs from that organ, and resembles the outer branch of a mesosomatic limb of Limulus.

On the whole, it must be admitted that  supplies a few more links to the chain of evidence pointing to the descent of the scorpions from marine Limuloid ancestors.

The figure gives a magnified view of the specimen of, Pocock, from the Upper Silurian of Lesmahago, Lanarkshire, It. was formerly in the collection of Dr. Hunter, and is now in the Kilmarnock Museum. This lithograph was executed from the specimen itself by Miss G. M. Woodward, under the supervision of Mr. R. I. Pocock, in October, 1900.

I—VI. Prosomatic appendages. st. Sternal area of prosoma. gen. Genital cleff. pect. Pecten, or external branch of appendage of second somite of mesosoma. end. internal branch of appendage of second somite of mesosoma. ''sup. lat., inf. lat., inf. med.'' Superior lateral, inferior lateral, and inferior median crests of fourth somite of metasoma.