82 



Male of about the same size as the female, and rather similar in appearance, 

 though having the urosome narrower and divided into 5 well-defined segments. 

 Eight anterior antenna with the middle section but slightly tumefied. Last pair 

 of legs with the outer rami very unequal, that of left leg much the longer, with 

 the distal joint twice as long as the proximal one, that of right leg with the 

 distal joint quite short, club-shaped, and carrying at the tip, outside the spiniform 

 process, a very small bidentate piece (rudiment of a 3rd joint). 



Colour. Body in both sexes highly pellucid and nearly hyaline, though, 

 as a rule, exhibiting within the metasome a rather large oblong oil-bubble of a 

 clear orange hue. 



Length of adult female reaching 2.20 mm.; that of male about the same. 



Remarks. This form, which, being the first one described, may be regarded 

 as the type of the genus, is very closely allied to the marine species, L. Grimaldii 

 (de Guerne), with which it has been confounded by earlier authors. On a closer 

 comparison, however, it is at once distinguished by the peculiar shape of the 

 cephalosome, as seen laterally. Whereas in L. Grimaltlii the dorsal margin of 

 this part, as in most other Calanoids, forms an uninterrupted curve up to the 

 rostral prominence, in the present form it is conspicuously insinuated in the 

 middle, bulging out in front of the sinus almost in a gibbous manner, to join 

 the rostral prominence in a very oblique curve, thus giving the frontal part of 

 the body a very peculiar appearance. The size of the present form is also rather 

 inferior to that of the marine species. 



Occurrence. I have found this form very abundantly in some of the larger 

 Norwegian lakes, viz., Mjosen, Tyrifjord, Randsfjord, Storsjo and Femsjo. It did 

 not occur in any of these lakes except at some distance from the margin, and, 

 as a rule, not at the very surface of the water, but only at some depth below 

 it. In all the specimens collected (during the summer months) a large orange- 

 coloured oil-bubble was constantly found within the metasome, whereas the ovarial 

 tubes appeared, as it were, skrunk, so as to be detected only with great difficulty. 

 This circumstance, in connection with the fact that all the specimens taken were 

 fully grown, seems to prove that the propagation of this form must be confined 

 to a different season of the year, perhaps the winter. It has been conjectured 

 that this Calanoid might more properly be regarded as a relict marine form, a 

 supposition which, however, was chiefly based upon the erroneous confusion of the 

 2 nearly-allied species, L. macrurus and L. Grimaldii. I think, however, that we 

 must admit a near genealogical relationship between these 2 forms, the former 

 being in all probability a direct descendant of the latter, though at present it 



