The Long Island Cyclops (C. insignis?), having fourteen-jointed antennæ, three-jointed rami in the swimming feet, with two-jointed fifth feet and elongate caudal stylets, is a transitional stage in the development of a seventeen-jointed form C. parcus (Herrick ?). The eighteen-jointed antenna is derived from the seventeen-jointed form by division of the seventh segment. Out of fifteen individuals taken at random, none of whose antennal segments exceed twelve, five precisely similar individuals constitute a group having nine antennal segments, two-jointed rami and two-jointed fifth feet. Four individuals constitute a second group having typically ten antennal segments, two-jointed rami, and two-jointed fifth feet. Two of these individuals show marked variation, one in having three-jointed rami in the swimming feet, the other in having a one-jointed fifth foot. Four individuals constitute a third group, characterized by eleven-jointed antennæ, three-jointed rami, and two-jointed fifth feet. One member of this group has a one-jointed fifth foot, and this is the only individual out of the thirteen that can be given any place among species, i.e.,C. phaleratus, as combining well recognized species-characters. Three individuals constituting a fourth group combine the following characteristics: twelve-jointed antennæ, three-jointed rami, and two-jointed fifth feet. These forms suggest C. varicans, with which they have much in common, but from which they differ considerably in detail. Some facts point to the probability that the Cold Spring Harbor forms with the ten-jointed antennæ are morphologically undeveloped. Especially does the variation within the group consisting of but few individuals point to the instability of these forms. What the true nature of these correlated peculiarities in Cyclops may be, can only be determined by following the life history of each individual. The relatively large size of these forms, and the frequency with which they occur, as well as the constancy of the correlated characteristics, suggest on first acquaintance with the Cyclopidæ, that they represent distinct species, but a fuller acquaintance warns us to look further for an explanation of these most perplexing variations which are doubtless largely due to the acquiring of sexual maturity while the morphological changes in the body are still incomplete, and to the varying external conditions to which they are subjected.