﻿32 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



possible that these animals really do not live in that area, but perhaps 

 it would be premature to regard this as an established fact. We must 

 not forget that notostracans are very easy to overlook even in places 

 where much general collecting is done, and Lepidurus is found in other 

 parts of the world, such as southeastern Australia and New Zealand, 

 where the climatic conditions are similar to those in the eastern United 

 States. The western part of North America, from Alberta in the north 

 to Arizona in the south, in spite of the relative smaUness of the area, 

 and the comparatively i&w localities from which material is reported, 

 presents a richness of forms unparalleled in other parts of the world. 

 Four species are found here as against two in boreal Europe and Asia, 

 one or possibly two (others are certainly synonyms only) in Australia, 

 and one in South America. Species with a large number of leg-bearing 

 rings {L. hilohatus and the new species of Lepidurus) have not been 

 found anj^where else in the world. 



It has been advanced that there is a biological difference between 

 Lepidurus and Apus, the latter occurring in pools which regularly 

 dry up, while the former are found in permanent waters (Brauer, 

 1878, and Barnard, 1929). However, I have observed L. apus in 

 several ponds in Sweden which dry up every year. Further, Brauer 

 states (p. 587) that species of both genera sometimes live in the same 

 pool. Perhaps the eggs of Lepidurus may be able to stand some 

 desiccation, though those of Apus certainly can stand more, or perhaps 

 the different species of Lepidurus have varying abilities in this respect. 



Both Lepidurus and Apus are recorded from Montana, Oregon, 

 California, Nevada, Utah, Colorado, and Arizona, though they are 

 not known to occm* together in the same pool there. It is not known 

 what differences there may be between the respective localities. 



KEY TO THE NORTH AMERICAN SPECIES OF LEPIDURUS 



A. Leg-bearing abdominal rings 9.5-13, inclusive, front part of nuchal organ 



between hind part of eye-tubercles, not far from hind margin of eyes. 



a. Endites 3-5 of first leg rather similar in size, very little or not at all pro- 



jecting beyond margin of carapace arcticus (p. 33) 



b. Endites 3-5 of first leg much dissimilar in size, fifth endite of first leg 



clearly projecting beyond margin of carapace. 



1. Mediodorsal spines on supra-anal plate not on a keel, few in number, 



rather similar in size packardi (p. 33) 



2. Mediodorsal spines on supra-anal plate on a distinct keel, numerous 



(20-100), and highly variable in size couesii (p. 34) 



B. Leg-bearing abdominal rings 14.5-18, inclusive. 



I. Front part of nuchal organ between hind parts of eye-tubercles, not far 



from hind margin of eyes bilobatus (p. 38) 



II. Nuchal organ considerably behind eye-tubercles. 



a. Hind part of lateral margin of carapace with minute spines, lynchi (p. 39) 



b. Hind part of lateral margin of carapace with large spines, directed 



straight outward lynchi var. ecliinatus (p. 49) 



