FAMILY SPHHROMIDA. 95 
It is well known that the end of abdomen is shaped very 
differently inthe genera. In Spheroma, Hemispheroma, 
and the section Monolistrini the posterior margin of abdo- 
men is broadly rounded, without trace of longitudinal excava- 
tion below or of any terminal notch. In other genera the 
lateral walls of the terminal part of abdomen are bent less or 
more downwards and sometimes even a little inwards, so that 
the lower side shows a longitudinal excavation (Isocladus), 
and when in this case the end of abdomen is cut off we have the 
dorsal half of a kind of tube (Ancinus). In Cymodocella 
the distal lateral walls mentioned are so strongly curved that 
their lower margins touch each other below in the mesial 
line; the lower distal surface of abdomen is in this case the 
inner wall of a tube formed by that curvature, and the tube 
terminates behind in a nearly circular aperture. In other 
genera the end of abdomen has a real notch; sometimes this 
notch is very deep, its distal portion narrowed, being only a 
linear slit, while the proximal part is a rounded or transverse 
foramen. Such differences have been seen and described by 
all authors ; they have generally been used as specific char- 
acters, but they are always of generic value; nobody seems to 
have noticed that the want of a notch or the essential shape 
of the notch is of importance as to the biology of the animal. 
The best instances are the genera Leptospheroma (Hilg.) 
and Plakarthrium (Chilt.). In these forms the uropods 
surround the end of abdomen ; the animals are very depressed, 
with the lower surface concave, the outline continuous, and 
all parts participating in forming the outline are much 
expanded. According to Chilton Plakarthrium typicum 
(Chilt.) lives on the seaweed Eklonia radiata, “to which it 
closely adheres.” Both genera are evidently adapted for 
clinging closely to firm and flat or regularly rounded surfaces 
just as is a female Coccus ona Nerium. The end of abdo- 
men terminates in Plakarthrium in a notch; in Lepto- 
spheroma the most distal small portion of abdomen is 
turned somewhat upwards and has a longitudinal groove 
below; in both genera a small aperture is thus formed 
