new British Entomostraca. 241 
Colour as in female. 
This species resembles J. rapiens (Schmeil) very closely, 
but is easily distinguished by its greater activity and by the 
two-jointed inner ramus of the first leg. It differs also in the 
following points :— : 
(1) In JZ. rapiens there is a conspicuous circular mark * 
laterally on the cephalothorax and on the first two free 
thoracic segments, whereas in JZ. estuarid such a mark is 
only found on the first free thoracic segment. In both species 
there are similar marks on the second and third abdominal 
segments, but in MZ. rapiens they are confluent ventrally. 
(2) The anal operculum of the female is smooth in JZ ra- 
piens, but denticulate in MW. estuarti. 
(3) The furcal rami of the female are more constricted 
distally in M. rapiens., ; 
(4) The number of spines and setz on the swimming-legs 
is not the same in the two species, as shown in the following 
table :— 
M. rapiens. | M. estuarit.| M., rapiens. | M., estuarii. 
Female. | Third joint of outer ramus. {Second joint of inner ramus. 
| 
Spines.| Setae. |Spines., Sete. Spies Setze. |Spines,| Seta. 
| 
Lep'2..3 | 3 2 a a 1 4 | 4 
| | 
Bees h).'|) 3 4 Pa ake: 1 4 1 4 
| Leg 4 Ce, Pe 2 4 1 4 1 4 
(5) In the third leg of. the male of J. rapiens the outer 
ramus is considerably modified, whereas in M. estuarii it 
does not differ from that of the female. 
(6) The fifth legs of the female are scarcely distinguish- 
able, but those of the male are quite distinct. 
A number of individuals of this new form were met with 
in mud scraped from wooden piles near Acle Bridge in the 
River Bure in Norfolk. The great activity of the animal 
was very noticeable. The majority of the individuals taken 
* My attention was first drawn to these striking “species marks” by 
Mr. D. J. Scourfield, 
