18 Mr. H.J. Carter on Freshwater Rhizopoda 
The last two or three normal segments in Insects (that is, the 
16th, 17th, and 18th) are frequently wanting. 
In the above homological comparisons it is assumed that the 
three anterior normal segments present in a Crustacean are 
normally and potentially present in an Insect. This will be con- 
sidered by many as the doubtful point in the above comparisons. 
But it is proved to be correct by the fact that these three seg- 
ments are sense-bearing segments in Crustaceans, and the Insect 
fails in no sense belonging to the Crab. As stated on page 188 
(Annals, /. c.), the absence of a jointed organ is no proof of the 
absence of the segments, unless it be true also that the corre- 
sponding sense is wanting. 
If the constitution of the anterior part of the head in the 
Insect be still questioned, there is nevertheless good reason for 
making the mandibular segment in the Articulate type (as it ad- 
joins the centre in embryonic development, from which progress 
goes on forward and backward) normally identical in all groups 
under that type; and hence from this segment, or No. 4 in the 
Crustacean series, on to No. 18, the parallelism between the 
Insect and Crustacean must be rightly given; consequently, if 
there is any doubt, it holds only with regard to Nos. 1, 2, and 3. 
The law of unity of structure under a type seems, however, to 
preclude even this chance for doubt. 
Comparing the higher Decapods among Crustaceans and the 
higher Insects, the mean size or mass is about as 50 tol. This 
ratio indicates approximately the amount of condensation in the 
Articulate structure connected with the elevation of grade from 
the typical Crustacean to the typical Insectean. 
I1I.—On Freshwater Rhizopoda of England and India; with 
Illustrations. By H. J. Carter, F.R.S. &e. 
[Plates I. & II.] 
Tue object of this communication is to bring together descrip- 
tions and illustrations of the freshwater Rhizopoda which I have 
found in England since my return from India in 1862, and also 
to add some of those which from time to time came under my 
notice in the island of Bombay, in order that I may, to a certain 
extent, show what species are common to both localities, and 
also some of those which may be peculiar to Bombay, or at least 
may not yet have been described. 
Further, I have drawn most of these on the same scale, and 
have added some of their varieties respectively, so that an idea 
may be thus gained of their sizes generally, as well as of some 
of their differences in point of shape ; for, unless they are treated 
