great exterior similarity of the eurypterids and ostracophores and the explanation of this phenomenon as resulting from adaptation to like conditions. The ostracophores have been generally regarded by paleontologists as owing their peculiar form to their mud-grubbing habit, and it may be inferred that the eurypterids, being of similar form, were of like habit and perhaps of like form because of similar habit.
III
GEOLOGICAL DISTRIBUTION AND BIONOMIC RELATIONS
In this chapter we shall first survey the geological distribution of the eurypterids in North America as indicated by the following conspectus, compare this distribution with that in Europe, and finally attempt a conclusion as to the physical conditions under which these strange creatures lived.
A Conspectus of American species arranged according to their geological occurrence
Algonkian
Beltina danai Walcott. Greyson shales, Montana
Cambric[1]
Strabops thacheri Beecher. Potosi limestone, St François county, Missouri
- ↑ There occur gigantic tracks in the Potsdam rocks of New York which, have been considered by good authorities as suggesting the presence of merostomes at that age. These tracks known as Climactichnites, were first described by Logan [Can. Nat. & Geol. 1860. v. 5] and later recorded by Hall [N. Y. State Mus. 42d Rep't. 1889. p. 25] from Port Henry, Essex co., N. Y., and by Woodworth [N. Y. State Mus. Bul. 69. 1903. p. 959] from the town of Mooers, Clinton co., N. Y. In the latter locality they assume gigantic proportions, being 6 inches wide and 15 or more feet long, terminating in an oval impression 16 inches long.
Various explanations have been suggested for these tracks. Besides having been referred to trilobites, burrowing crustaceans, plants, gastropods and annelids, they have been compared with those of the horseshoe crab, first by Dawson and recently again by Hitchcock and Patten. Sir William Dawson [Can. Nat. & Geol. 1862. 7: 271], who studied the American Limulus on the seashore, pointed out that when Limulus creeps on quicksand, or on sand just covered with water it uses its ordinary walking legs and produces a track strikingly like that described as Protichnites from the Potsdam sandstone, but in shallow water just covering the body, it uses its abdominal gill plates and produces a ladderlike track the exact counterpart of the
