120 Records of the Indian Museum. [Vou. Vil, 
of a circular distribution, the internal parts of which have been 
obliterated by the mighty development of younger and stronger 
forms, in this case the vigorous genus Pherettma, which, from 
Burma to New Hebrides in one direction and Japan in another, 
has suppressed and partly exterminated all other genera of earth- 
worms, those of its own phylum or sub-family as well as those of 
other tribes.” 
It is, however, difficult to bring the case of X. curvirostris in 
line with this view. There is no evidence that X. compressa repre- 
sents a young and vigorous type which has exterminated its near 
ally in localities lying between Assam and New Zealand; on the 
contrary it would rather seem that both species are archaic forms 
that must have arisen almost simultaneously and, while it is by 
no means impossible that Caridina may have suppressed X. curvi- 
vostyis in Eastern Asia, it is difficult to see why the same cause 
should not have effected its destruction in Assam. 
The genus Xiphocaridina is unquestionably a very primitive 
one and it may be predicted that such forms are less lable to 
evolve varieties, local races or other species than those exhibiting a 
greater degree of specialization.' That this is so is indeed self- 
evident, for a primitive form, if it be primitive, must necessarily 
have existed without considerable modification for a prolonged 
period and the mere fact that it has done this is an indication that 
it is less likely to adapt itself to any altered conditions of its 
environment than is a form which by its very specialization 
showed that in the past it had given a more ready response to 
such changes. 
The full significance of the unchanged condition of X. curvi- 
vostrts is, indeed, only realized when the great range of variation in 
certain other Atyidae is considered. Caridina nilotica is a species 
of wide African and Asiatic distribution. Specimens found in 
Bengal differ in certain measurable features from the type which 
occurs in Egypt and Dr. de Man has distinguished them under 
the name of C. ntlotica var. bengalensis. Among other varieties 
of the same species it agrees most nearly with var. gvacilipes 
found in Celebes and Salayer Is. Even within the limits of 
India and Ceylon, however, the form exhibits a most remarkable 
tendency to split into races, and series of specimens from Calcutta, 
Madras, Tuticorin, Ceylon and the Andamans each seem to possess 
its own particular characteristics. 
| Pocock (1889) has described several species very closely allied to Xiphocaris 
elongata ; but subsequent authors have preferred to regard them merely as varieties. 
I am, however, of the opinion that these forms are not deserving even of 
varietal recognition and believe that the suggestion which Pocock himself made, 
that they only represent stages in the growth of a single species, is likely to prove 
true. The rostra of large specimens of Xiphocaridina curvivostyis are asa rule 
relatively shorter than in smaller examples, and this is also the case with several 
species of Carvidina. In these instances, however, the variation has not nearly so 
great a range as in X. elongata. On the other hand there appears to be some 
evidence that two distinct races of X. compressa exist on Norfolk Is. (see Thomson 
1903, p. 449, and Grant and McCulloch, 1907, p. 151). 
