Page:The Habitat of the Eurypterida.djvu/134

 It is readily seen that a startling conclusion must be drawn from the data, namely, that the Llandovery is not a time period separate from the Tarannon, but that the two are synchronous, the Llandovery being equal in age to the lower Tarannon and appearing as a wedge which widens southward till it reaches its maximum thickness of 96 feet in the Moffat region. The Llandovery is a black mud facies containing a mixed Ordovicic and Siluric fauna, the evidences of the presence of the latter being indicated by the numerous species of Monograptus, the Ordovicic aspect being supplied by the Didymograptus species.

The terrestrial origin of the Tarannon has been shown by two different and mutually independent lines of evidence. It is of interest, then, to find in the Upper Tarannon the fragment of a eurypterid. Near the southern border of the Central Belt just south of Bowden which is northeast of Selkirk, there is recorded the occurrence, in the grey blue shales and flagstones probably of the Hawick series, of the telson of Eurypterus and a fragment of Dictyocaris associated with crinoid stems. The typical Hawick rocks found in the neighborhood of Selkirk and further south at Hawick are themselves barren of all fossils but trails, burrows, and tracks. Near Selkirk Crossopodia and Myrionites have been found, while at Hawick Protovirgularia, Crossopodia, Menertites, Nereites and other tracks are abundant and the body segments of a Ceratiocaris have been found. The occurrence of the single eurypterid fragment in this great barren series is difficult to explain as a marine organism. It may be argued that the presence of crinoid stems is clear enough evidence of the marine nature of the deposits, but such disjointed stems might be washed out from an earlier deposit or even if of the same age as the eurypterid it is well known that those joints are swept great distances from the original habitat of the crinoids, and that they might be washed far inland on low-lying flats along the shore. At any rate, the single eurypterid remain fails to prove anything definite; it might be washed in from the sea, but then one must ask why the eurypterids are not found in the Tarannon muds in the regions where abundant graptolite faunas have been found. The fluviatile origin of the Tarannon has been amply shown, and it is easy to understand on the supposition that the eurypterids were living in the rivers, that fragments of the exoskeletons should be washed out from time to time. It may here be suggested that a further careful search in the Tarannon rocks might well yield a eurypterid fauna as fine and as