ISOPODS 293 
logged timber, which without their destructive agency might become 
serious obstructions to navigation. Limnoria is only the fifth of an inch 
in length, and its back is covered with minute hairs, to which dirt usually 
adheres. It burrows with its mandibles, or jaws, which are chisel-like 
at the ends. Its habitat extends from a little above 
to a little below tide-marks throughout the whole At- 
lantic coast. 
GENUS Spheroma 
S. quadridentatum. The name of the genus 
to which this species belongs is derived from the 
peculiar habit of many of the species of rolling them- 
selves into a ball when alarmed. The body is so 
constructed as to admit of this singular change of 
shape. The abdomen turns under, and the last 
Spheroma quadridentatum. abdominal appendages, together with the narrow 
epimera, shut in the legs and cover the ventral 
portions with armor. The body, when extended, is an ellipse a little 
over a quarter of an inch in length and half that in breadth. The legs 
are hairy and adapted to walking. The anterior abdominal segments 
are fused into one, but are marked at the sides with depressed lines. 
The abdominal feet are plate-like and fringed with hairs. A slight 
elevation runs around the margins of the animal like a border. The 
color is variable. Some are a uniform slaty gray; others are marked 
with a longitudinal patch of color on the 
back. It is found from Cape Cod to Florida 
among alge or rocks, and is easily recog- 
nized by its habit of rolling itself into a 
ball.. Spheroma destructor is a boring 
isopod, larger than Limnoria, and is even 
more destructive, since its holes are larger ; 
but its range is limited, or, at least, it has so 
far been found only in the St. Johns River, 
Florida. 
Genus Idotea 
I. marina (Linneus), I. irrorata 
(Edwards). This species is about one inch 
long, and is easily recognized by the ab- 
domen, the first three segments of which are 
narrow and terminate in acute teeth, while the 
other three are fused into one with straight 
sides and ending in three teeth, of which 
the middle one is thelongest. The first pair 
of abdominal feet are large, long, and plate- 
like, covering the other'feet and whole under 
surface of the abdomen like an operculum. Idotea marina. 
The head is nearly square, the eyes are ; ; 
small, and the antenne have long peduncles. The articulations of the 
thoracic feet are fringed with hair. The color of this species varies: 
sometimes it is light or dark green, or brown with black spots; again 
