316 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



The telson is produced behind, with a long, broad, often spatalate 

 plate or expansion, in A. glacialis, twice as long as the body of the tel- 

 son itself; the end subacute, or broad and rounded, or bilobed; the 

 edge smooth or spiny. The telson itself less spiny than in Apus. The 

 caudal stylets, or cercopoda, about as in Ajms. 



The principal generic differences are in the long produced telson, the 

 shorter endites, the usually larger carapace, being larger in ])ro}>ortion 

 to the body than in A^nis (though not so in L. Mlobatus), and with usu- 

 ally only from 5 to 12 abdominal segments, besides the telson, projecting 

 beyond the hinder edge of the carapace. 



Geographical distribution. — Western IS^orth America appears to be 

 richer than Europe-Asia in the species of this genus, one [L. pro ductus) 

 occurring in the Eastern Hemisphere, ranging from Central Europe to 

 Scandinavia and England, while two species occur in the central zoo- 

 logical province of the United States, and one in Greenland and Arctic 

 America. No species of the genus have as yet occurred in the Missis- 

 sijjpi Valley or on the Atlantic coast, and none on the Pacific coast. 



Synopsis of the species. 



A. Endites 2-5 very short, not projecting beyond the carapace. Car- 



apace large ; telson short, pointed, si)iny on edge L. glacialis. 



B. Endites 2-5 rather long and slender, projecting w^ell beyond the car- 



apace. Carapace large, leaving only 5 abdominal segments and 



telson uncovered ; telson long, spatulate L. couesii. 



Carapace very short ; telson long, bilobed L. hilohatus. 



Lepidurus glacialis Kroyer. 



Plates XVI, tigs. 1 (enlarged nearly 3 times), la, 11) ; XVII, figs. 1, 5; XXI, figs. 1, 2. 



L. (jlaciaUs Kroyer. NaturMstorisk Tidskrift, Sd ser. vol. ii, 431. 1847. 



Carapace very large, narrowing somewhat toward the eyes, being 

 more regularly ovate than in L. couesii^ whicli is more elliptical ovate. 

 The twelv^e terminal abdominal segments aie left uncovered by the car- 

 apace ; the telson broad at the base and extended into a blunt prolonga- 

 tion armed with coarse teeth on the edges, and as long as the telson is 

 broad at base; three sharp median teeth and a finely-denticulated tu- 

 bercle on each side, at the base ; the telson and its extension are smooth 

 beneath. The cercopoda or caudal stylets are nearly as long as the 

 body, slender and very hairy, rather than spiny. 



The appendages differ decidedly from the other American species in 

 the 2d to 5th endites being very short and broad and more equal in size ; 

 the 5th endite is much shorter than in the other species ; the third and 

 fourth of nearly the same size and length, and one-half as long as in the 

 two other species ; the second is about twice as large in proportion as 

 in the two other species. The scale of the sixth endite is very long and 

 slender, the tip much attenuated, with very long, hair-like setoe ; the gill 

 itself narrow, pear-shaped. 



In the second pair of feet the second endite is twice as large as in 

 the two other American species; the third and fourth of about the 

 same size as in the other species, while the filth is about one half as 

 long, the scale (6th endite) very large and ensiform, with the tip curved 

 and ending in a spine, the inner edge with sharp spinules, the outer edge 

 wath numerous long hairs. The accessory gill is inequilaterally triangu- 

 lar, the proximal edge straight, not produced backwards, as in the other 



