FAMILY GAMMARIDAE. 
Both antennae well developed, the upper long, slender, filiform, usually 
immediately above the lower, which are inserted into a notch at the front 
angle of the head. First and second feet sub-chelate. Eyes compound, 
commonly between upper and lower antennae. 
Genus Gammarus. 
No rostrum. Three last abdominal segments each with two or more 
clusters of short stiff spines on hind margin. Secondary flagellum and man- 
dibular palpus present. Last pair of abdominal legs two branched: telson 
double. 
G . fasciatus , Sat/. Secondary flagellum as long as second segment 
of peduncle (19), and composed of five or six segments. Fourth, fifth and 
sixth abdomi na l segments each with three clusters of spines on hind margin. 
Genus Crangonyx . 
No clusters of spines on posterior abdominal segments. Telson single ; 
last pair of abdominal legs with inner branch rudimentary or wanting. Pe- 
duncles of the two pairs of antennae sub-equal. The first two pairs of 
feet sub-equal. 
C. gracilis , Smith. Eyes evident. Hind angles of first three abdom- 
inal segments each ending in a sharp tooth. Outer branch of last pair of 
legs about twice as long as peduncle ; inner branch very small. Telson 
short, emarginate. 
C. mucronatus , Forbes. No eyes. Hind angles of first three abdomi- 
nal segments rounded. Outer branch of last pair of legs shorter than pe- 
duncle, inner minute. Telson of male a slender spine about as long as 
first three abdominal segments. 
ORDER ISOPODA. 
Body commonly depressed ; thoracic segments not consolidated ; eyes, 
if present, compound, sessile. Gill plates beneath abdomen. The last 
four pairs of thoracic legs similar, and differing from the first three pairs. 
Last pair of abdominal legs more or less styliform. 
FAMILY ONISCIDAE. 
Abdomen many-jointed, last segment small, caudal stylets (20) well ex- 
serted. Mandibles without palpi. Inner antennae obsolete. 
FAMILY ASELLIDAE. 
Body very flat, loosely jointed. Last abdominal segment very large, 
shield-like, comprising nearly the whole abdomen. Upper antennae short, 
lower very long. Only first pair of feet sub-chelate. Mandibles with 
palpi. 
Genus Asellus. 
First pair of feet sub-chelate ; last thoracic legs not elongate ; first 
pair of abdominal appendages in female (first two pairs in male) small, 
forming short plates; outer ramus of next pair serving as gill-covers; cau- 
dal stylets elongate. 
