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 ccecca slender, Antenne weak. The typical form of head is 
that of Fig. 3, Plate LIII, 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 sete, 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 dubia Herrick.) 
PLATE XLIX, Fias. 7, 8. 
Our D. dubia 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but the head is strongly crested all around and the eye is withdrawn, 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 Daphnia. The 
form is as in D. pellucida, 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. Ina 
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 
eineeneens 
