Page:The Habitat of the Eurypterida.djvu/251

Rh such pronounced ears on the epimera, although these are extended posteriorly to a much more pronounced degree, than in S. logani.

The only species which has epimera approaching in size and form those of S. macrophthalmus is S. scoticus from the Old Red sandstone (see p. 251 below), from which, however, it differs in certain important features. It is closest to a second species found in the Old Red sandstone, S. powriei, which it resembles in the tapering form of the body the long, narrow telson, the subquadrate outline of the head (this is decidedly square in logani) and in the great length of the fourth appendage. In details, on the other hand, these two species differ considerably, so that S. logani must remain a rather separated species until new discoveries reveal its relatives. It is of great interest to have reported from the Ludlow fish bed in one of the tributaries of Greenock Water (see p. 164 above), Stylonurus ornatus associated with the typical Lanarkian (Downtonian) fishes and with Eurypterus dolichoschelus, a Ludlow and Lanarkian species, together with Ceratiocaris, Dictyocaris, plants, etc. (p. 164 above). S. ornatus, then, evidently persisted from Wenlock into and through Ludlow time. In this case one is again confronted with an anomalous geographic and geologic distribution. The Pentland Hills are less than thirty miles distant from the Lanarkian inliers, and the two areas are approximately on the same line of strike. In two thin beds but a few inches in thickness and extending only a few yards laterally S. ornatus occurs in the Wenlock in Lanarkshire; but in the Pentland Hills this species occurs in none of the many Ludlow eurypterid horizons until the fish bed is reached and there a few specimens are found. If the eurypterids lived in the Wenlock sea as they are commonly supposed to have done, then the supporters of this view must account for the limited vertical and horizontal distribution of the merostome remains, since it is absolutely inconceivable that members of a marine neritic fauna should be confined to an area a few square yards in extent. It is equally inconceivable that a marine fauna should be perpetuated for so great a period of time as from the Wenlock through the Ludlow, the members of the later fauna in some cases showing resemblance to members in the earlier, while in others they are entirely distinct and apparently arise suddenly, there being, besides, no indications of a persistent marine stock to furnish decendants from the Wenlock fauna, nor yet any trace in the maine Ludlow of the incursion from other regions of new genera and species