574 GILBERT ©. BOURNE. 
give a satisfactory representation of the number of abdominal 
segments in his figure, which otherwise resembles some of 
Sinel’s specimens from Jersey, which I have identified as 
M. anglica. 
The young specimens found in Plymouth in August by 
Dr. Norman have three setz on each furcal member and two 
abdominal segments; they are identical with a single adult 
female in the collection sent to Dr. Norman by Sinel, which is 
further characterised by the shape of the first abdominal 
segment, which is elongate, twice as long as the last thoracic 
segment, and swollen, and by the fifth pair of legs, which are 
elongate, and have relatively long endopodites. These are 
clearly identical with Thompson’s Cymbasoma rigidum, 
and must be placed under a separate species as M. rigida. 
Finally there is the single specimen taken by myself at 
Plymouth on Sept. 11th, which differs from all other species in 
the immense length and single origin of the genital appendage, 
and from M. rigida in the proportions of the fifth pair of legs 
and of the first segment of the abdomen. This specimen I 
propose to describe as a new species under the name of 
M. longispinosa. 
To sum up, the genus and species of Monstrilla may be 
defined as follows: 
Genus Monstrilla, Dana. 
Cephalothorax composed of five apparent segments ; the first 
segment consisting of the fused cephalon and first thoracic 
segment, enormously elongated, covered with a delicate trans- 
parent chitinous shield, often separate from the body wall. 
Abdominal segments 2—4, besides the furca. Four pairs of 
powerful thoracic swimming feet, a fifth pair, rudimentary, 
present in the female. The second antenne and all the 
gnathites absent in the adult. The females provided with a 
genital appendage on the ventral side of the first abdominal 
segment, in the form of two sete or a single seta which 
bifurcates at some distance from its origin, and which may 
greatly exceed the length of the whole body. Male with an 
