158 THE HABITAT OF THE EURYPTERIDA 



is that the Lingulae are found in abundance in the marine littoral 

 fauna where they occur, normally associated with marine species of 

 molluscs, Crustacea, etc., in the marine deposits of the same age 

 further south; furthermore, Lingulae are found from the Cambric 

 to the present in undoubted marine associations. The eurypterids, 

 on the other hand, are not found in the unequivocal marine deposits 

 to the south, but appear quite as suddenly as the Lingulae, although 

 in separate bands. They have been found to the north of the Ludlow 

 area in Scotland, always as concomitants of the transition from ma- 

 rine to continental conditions, and it is only when the latter condi- 

 tions transgress farther and farther south that the eurypterids appear. 

 I think that much light will be thrown upon the interpreta- 

 tion of the late Siluric deposits in England by the study of the Lud- 

 low and higher bone-beds. It will not be possible in this paper to 

 consider the habitat of the early fishes except incidentally, but if 

 that is proved to be fluviatile, as I think it may be, then the follow- 

 ing explanation may be offered for the bone-beds. The Ludlow 

 Bone-Bed, which is the most constant and widespread, appears to 

 mark the wholesale destruction of the fishes in the rivers at the time 

 when, in the oscillatory movements preceding and accompanying the 

 retreat of the sea, there were temporary advances. The salt water, 

 pushing its way up the rivers, killed the fishes and other river organ- 

 isms in great numbers, for the fluviatile fishes can less easily survive 

 an influx of salt water than marine fishes can an influx of fresh water. 

 This is implied in Giinther's statement that "On the whole, instances 

 of marine fishes voluntarily entering brackish or fresh water are 

 very numerous, whilst fresh-water fishes proper but rarely descend 

 into salt water" (97, 187). Thus during the oscillations preced- 

 ing a negative eustatic movement, the sea would occasionally ad- 

 vance a short distance over the land, and if this temporary positive 

 movement were widespread, bone-beds would be formed at or near the 

 mouths of many rivers almost contemporaneously, and even if some 

 areas were submerged and others not, geologically the bone-beds 

 would appear to be approximately synchronous. This theory is borne 

 out by the occurrence of thin bone-beds at a number of higher levels 

 in the beds above the Ludlow Bone-Bed. Moreover, whenever there 

 was a slight retreat of the sea with the pushing forward of terrigen- 

 ous, coarse material, then the light Lingula shells might well be 

 left stranded along the line marking the high-water level for that 

 particular period. If such a negative movement were followed by a 



