DISTRIBUTION AND OCCURRENCE OF EUKYPTERIDS 519 



With these facts in mind, we may ask the pertinent question, Where 

 did this great colony of Eurypterids originate? If this was a lagoon or 

 an estuary and the Eurypterids constituted an estuarine fauna, this must 

 have been derived either from the sea or from the land waters. Estuarine 

 faunas, though possessing characters of their own, can be composed only 

 of animals derived from one or both the contributing realms, the sea or 

 the; land. No one would assert that a lagoon or estuarine fauna comes 

 suddenly into existence with the appearance of the restricted water body ; 

 that it has no progenitors in the sea or in the land waters which combine 

 to form the estuary. And yet it has generally been thought sufficient to 

 refer to these organisms as estuarine or lagoon types, with the tacit as- 

 sumption that that explained their presence. Clarke, speaking of the 

 Eurypterids of the Shawangunk, says that "our present knowledge of the 

 habits of the Merostome crustaceans, derived from both the living and 

 fossil forms, indicates the shallow water or barrachois origin of all sedi- 

 ments in which these remains abound," 119 but he does not discuss the 

 origin of this fauna, which he himself considers as normally absent from 

 the sea. 



It is evident that the Pittsford and Shawangunk deposits are inti- 

 mately associated in origin. Essentially the same species of Eurypterids 

 occur in the black shales of both, and the Pittsford can only be regarded 

 as the westward extension of the Shawangunk deposition. This is fur- 

 ther shown by the fact that in both cases red shale deposition follows the 

 Eurypterid beds. The Eurypterids of the Shawangunk shales are very 

 fragmentary, only the youngest individuals remaining intact. Billingsley 

 has observed, furthermore, that the fragments are much dissociated ; that 

 the cephala, for example, are by themselves as if swept together, while 

 elsewhere the body segments occur. Young individuals are extremely 

 numerous, and these are as a rule well preserved. In the Pittsford shale, 

 on the other hand, the exoskeletons are commonly preserved in a good 

 state of perfection. This might at first seem an argument for the exist- 

 ence of the Eurypterids in the waters in which the remains are so well 

 preserved. We might ask, Why is it, if the Eurypterids were swept by 

 the rivers from the land, that the entire specimens were carried out into 

 the lagoons which existed before the final withdrawal of I he sea, while 

 only comminuted remains and young individuals were embedded in the 

 black muds of the supposed river delta? In attempting to answer this, 

 let us assume, for the sake of argument, that the Eurypterids are river 

 animals. During periods of flood the molted exoskeletons of old and 



New York State Museum, Bull. 107, p. 303. 



