392 GEOLOGICAL SURVEY OF THE TEEEITORIES. 



In the second and succeeding thoracic pair of limbs the second to 

 fifth endites are short and nearly equal in size, while the sixth is much 

 larger than in the first pair, being nearly as long as the fifth endite and 

 varying somewhat in the different species. In Lejpidurus glacialis it is 

 noticeably slender, as are the exites. 



In the male of A-pns dispar from the White Nile, the second pair of 

 feet are curiously modified to serve as grasping organs, the notches 

 along the edge of endites 2-5 being much enlarged so as to aid the ani- 

 mal in retaining its hold of the female. 



A more generalized form of the leg is seen in the tenth and several 

 succeeding i^airs (Plate XVIII, figs. 1-4 ; XXI, figs. 1, 3, 4, 5), there 

 beingno difference in form between the last thoracic (tenth) and first ab- 

 dominal (eleventh) legs ; except the female eleventh pair and the fact that 

 the eleventh male foot has the genii;.! pore. 



The tentii leg of Apus liicasanus, for example (Plate XVIII, fig. 3), or 

 of Lejpidurus glacialis (Plate XXI, fig. l),lias a portion or lobe of the 

 axis, which Lankester calls the subapical lobe, which does not even 

 exist in a rudimentary state in the first i)air of limbs in A. lucaaanus; 

 nor does it exist in A. cancriformis, and is not to be seen in the larval 

 limbs of A. lucasanus figured by Gissler, nor is it figured by Claus. 

 Lankester regards this lobe as present in the second pair of thoracic feet 

 of A. cancriformis and figures it, but states that it " is relatively small." 

 We have not noticed it in the second pair of feet of any species of Apm, 

 but have seen it in the second feet of Lepidurus hilobatus (Plate XVII, 

 fig. 6), where it forms a lobe at the base of the exites. 



In the tenth pair of feet of the different species of Apus this lobe be- 

 comes a large and prominent expansion situated between the base of 

 the sixth endite and the flabellum. (Plate XVIII, figs. 2, 3, 4, x, and 

 Plate XXI, figs. 1, 4, 5, no lettering.) The iinportance of this exital 

 lobe becomes apparent when we examine the moditiedlegsof the eleventh 

 pair of the female. The history of this lobe in Apus cancrUormis has 

 been well related by Professor Lankester, and an examination of our 

 American species shows that it is developed in all our species of Apus 

 and of Lepidurus, much as he describes in the Euro[)ean Apus. 



In the posterior teet this lobe finally becomes obsolete. 



Under the rather ponderous name oostegopod., Professor Lankester 

 describes the singular ovisacs or brooding-legs of the female and their 

 mode of origin, with which our own observations on the American Apo- 

 didas agree. On Plate XVIII, figs. 5, 6, 7 ; XXI, figs. 2, ti, are shown 

 the forms of the eleventh pair of legs in the female of Apus and Lepi- 

 durus. The ovisac as originally shown by Zaddach, and more recently 

 by Lankester, is formed by the great development of the subapical 

 l{)be, over which, as Lankester says, "the flabellum fits as a lid." Our 

 lettering on Plate XVIII, fig. 7, was put on two years ago when 

 making the drawings for the plate, and from hasty examination and 

 overlooking the minute gill, which, however, is figured in this drawing, 

 w^e supposed, with Gerstaecker, that the sac was tormedby the flabellum 

 and gill; but since the i)iate was figured Lankester's description has 

 been ijublished, and upon re-examination we have found the mouth of 

 the egg-sac* (Plate XXXI, fig. 5, os). 



*Brauer, in 1872, in his Beitrage zur Keimtniss der Pliyllopoclen, gives an account 

 of tlie mode of copulation in Apus. In the spring of 1871, a male was discovered 

 among twenty J. The male swimming towards a 2 turned under the 9 , placed itself 

 firmly on the dorsal shield of the same, so that the whole body assumed a curved, 

 almost humpbacked, position, and made repeated convulsive contractions. It then 

 attempted, by feeling around with the end of its body over the hinder edge of the 

 carapace of the $ , to reach to it, and then threw several times and very rajjidly the. 



