196 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



the margin and occasioning a prominence in the ventral outline or 

 retreating from it. Macula nigra small or almost non pigmented. 

 Gastric coecca slender. Antennae weak. The tyj^ical form of head is 

 that of Fig. 3, Plate LIU, but the outline may be evenly rounded. 

 The range of variation is well shown by the various figures. The 

 young have no horn on the head. The spine of the shell is nearly as 

 long as the whole animal in the young. The male of our form is 1.2 

 mm. long, excluding the spine which measures 0.47 mm. The flagellum 

 is a very little longer than the sensory sette, and there is a very 

 minute lateral flagellum. A peculiarity of this species is the scattered 

 thorny armature of the spine of the shell. There is but little change 

 in the form of head with age. The form of the last feet is peculiar. 

 The ephippium occupies comparatively a small part of the valves and 

 the spine becomes very short and quite smooth. The sexual period 

 occurs in September and October. 



(*Daphnia diibia Herrick.) 

 Plate XLIX, Figs. 7, 8. 



Our D. dnbia may prove distinct. It is nearly related to D. hyalina, 

 but the head is strongly crested all around and the eye is withdrawn, 

 in young as well as old specimens, toward the middle of the head. 

 This peculiarity is shared in this degree by no other Daplinia. The 

 form is as in D. peUucida, but the spine is more slender and directed 

 upward. The head is shaped much as in D. vitrea in the young, but 

 is much less prominent. The older form has a shorter and more 

 slender spine (none were seen in the ultimate or spineless stage). The 

 head is more evenly rounded, but still well crested. The abdomen is 

 very slender and the anal teeth diminish rapidly in size from below 

 upward. The claws are very short and armed down the whole length 

 with fine bristles. The abdominal processes are well united at the 

 base in old specimens, so that the second seems a small process of the 

 first. The shell is very transparent and the spine is very long. In a 

 young specimen the spine was 1.0 mm., the body 0.7 mm., and the 

 head 0.4 mm. In this specimen the spine was slightly curved, the 

 head elongate with a slight ridge in front. Another individual had 

 the spine 1.1 mm. long, while the remainder of the animal was 1.3 

 mm. This specimen also had a knife-like hyaline ridge on the crest, 

 which was obliquely truncate in front; it also had numerous summer 

 embryos in the brood sac. The spine was perfectly straight and but 

 slightly inclined upward. Older individuals have a rounded crest as 

 figured and no ridge. The spine is relatively somewhat shorter but 

 much more slender. The characters which most clearly distinguish 



