and the Mutations of certain Atyids. 379 



mination of species, is a more rapid and sometimes a surer 

 guide than the examination o£ morphological characters. 

 In my opinion, Ortmannia Henshawi is neither more nor 

 less than a form of Atya bisulcata, a form which has 

 the curious character of recalling the immediate ancestral 

 form of Atya. We have not here to deal with an ordinary 

 dimorphism, sexual, produced by season or locality : the 

 specimens of M. Ballieu were collected in the month of May, 

 1877, in the vicinity of Honolulu, perhaps with one stroke of 

 the net ; in both forms there are the same variations of size 

 and sex. Some females of Atya bisulcata are charged with 

 ova, whilst the females of the Henshawi variation have none; 

 but in another consignment, also made by M. Ballieu, the 

 females of this variation carry a remarkably large charge 

 of ova. 



I should not perhaps have hazarded the foregoing con- 

 clusion if the Museum material had not permitted me to 

 extend it to other quarters of the globe. 



In 1890, M. Alluaud collected in a torrent on the Amber 

 Mountain, in Madagascar, a small shrimp which presented 

 all the characters of the genus Ortmannia, but differed from 

 the modification Henshawi by specific characters ; latterly 

 the Museum has received from Sainte Marie, in Madagascar, 

 a small batch of shrimps *, in which examples of Atya and 

 Ortmannia absolutely resembling one another (setting aside 

 generic characters) were mixed. The specimens of the first 

 form appeared to me to be classifiable as Atya serrata, 

 Sp. Bate : those of the second resembled that from the 

 Amber Mountain ; they have all the specific characters of 

 Atya serrata, and represent certainly, in my opinion, a modifi- 

 cation of this species. This will be, if desired, the modification 

 AUuaudi of A. serrata. 



A. serrata exists also in the island of Bourbon, where 

 Maillard, about 1854, obtained three specimens, which are 

 now in the Museum. The modification AUuaudi of this 

 species was found, in 1893, by M. Alluaud in the ravines of 

 the mountains of Salasie and Helbour. Another specimen 

 was taken by M. Alluaud, in 1890, in Mauritius ; the typical 

 A, serrata has not yet been noted in this island, but one 

 cannot doubt its existence there as well as in Reunion. 



These modifications are of great interest, because they put 

 in evidence one of the mechanisms by which new types are 

 produced and definitely established through more primitive 

 types which may persist or disappear. 



* These shrimps were captured in a little rivulet near Sainte Marie 

 in October 1895, and were presented to the Museum by M. Edouard 

 Chevreux. 



25* 



