392 GEOLOGICAL SURVEY OF THE TERRITORIES. 
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 Lepidurus glacialis it is 
noticeably slender, as are the exites. 
In the male of Apus 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 pairs (Plate XVIII, figs. 1-4; X XI, figs. 1, 3, 4, 5), there 
being no 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 genital pore. 
The tenth leg of Apus lucasanus, for example (Plate XVIII, fig. 3), or 
of Lepidurus glacialis (Plate X XI, fig. 1), has 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 pair of limbs in A. lucasanus ; 
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 Apus, 
but have seen it in the second feet of Lepidurus bilobatus (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, v, and 
Plate XXI, figs. 1, 4, 5, no lettering.) The importance of this exital 
lobe becomes apparent when we examine the modified legs of the eleventh 
pair of the female. The history of this lobe in Apus caneri/ormis 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 Kuropean Apus. 
In the posterior feet this lobe finally becomes obsolete. 
Under the rather ponderous name oostegopod, Professor Lankester 
describes the singular ovisacs or brocding-legs of the female and their 
mode of origin, with which our own observations on the American Apo- 
dide agree. On Plate XVIII, figs. 5, 6,7; X-XI, figs. 2, 6, 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 
lobe, 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, 
we supposed, with Gerstaecker, that the sac was formed by the flabellum 
and gill; but since the plate was figured Lankester’s description has 
been published, and upon re-examination we have found the mouth of 
the egg-sac* (Plate XX XI, fig. 5, 0s). 
*Brauer, in 1872, in his Beitrage zur Kenutniss der Phyllopoden, gives an acconnt 
of the mode of copulation in Apus. In the spring of 1871, a male was discovered 
among twenty 9. The male swimming towards a 9 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 9, to reach to it, and then threw several times and very rapidly the 
