North American Species of Diaptomus. 137 



The collection from which the variant mentioned above 

 was taken, made in May, 1894, consisted almost entirely of 

 D. sanguineus. Collections from the same waters made in 

 July, 1896, did not contain a single individual of this species, 

 but D. siciloides Lilljeborg and D. pallidas Herrick, were 

 present in immense numbers. 



VARIATION IN D. SANGUINEUS FORBES. 



Plates XXIII. , XXIV., and XXV. were prepared before the 

 thesis work proper was undertaken and exhibit the results of 

 a study in variation. From these figures it will at once be 

 evident that D. sanguineus is an unusually variable species, 

 and without the intermediate forms the extremes might 

 almost be regarded as distinct. The specimens examined 

 were all from the collections of the Biological Station at 

 Havana, so that the variations are probably not so great as 

 they would be if widely separated localities were represented. 

 Especial attention was given to variations of specific char- 

 acters, and most particularly to the relative proportions. 



The second basal segment of the right leg of the male, 

 which is usually very broad, in fact one of the most charac- 

 teristic features of the male, is shown in PL XXIII., Fig. 2, to 

 be sometimes of very ordinary width, the other extreme being 

 shown in PI. XXIV., Fig. 4. The relative position and length 

 of the projection on the outer distal angle of this segment 

 also vary a great deal, the extremes noted being shown in PI. 

 XXIII., Fig. 1 and 2. 



The marginal spine of the outer ramus of the right fifth leg, 

 the position, relative length, and characters of which are of 

 specific value in most species, lacks such value almost entirely 

 in D. sanguineus. The extreme variation is shown in PI. 

 XXIV., Fig. 4, and PI. XXV., Fig. 3. 



The inner ramus of the right fifth leg, though always very 

 short, varies in length from that shown in PL XXIV., Fig. 4, to 

 that in PL XXIII., Fig. 2, on the latter of which is also shown 

 a rather clearly marked suture which is usually wanting. 



The variation in the size of the males is indicated by the 

 drawings of the fifth pair of legs. (See PL XXV., Fig. 3-5, 

 and PL XXIV., Fig. 4.) 



