44 Records of the Indian Museum. [Vol. XIII, 



In distinguishing the species the most important indications 

 are those derived from adult males. The form of the petasma is 

 a most reliable and satisfactory guide and, according to my ob- 

 servations, an absolute criterion of specific identity, while good 

 characters are also to be obtained from the sexual modifications of 

 the external flagellum of the antennule. In three of the four species 

 the ultimate segment of the antennular peduncle of the male is 

 always elongated to a very remarkable degree, a great contrast 

 existing in this respect between the two sexes. It is curious that 

 this very striking feature has not hitherto been noticed. In the 

 fourth species {A. erythraeus) the ultimate peduncular segment, 

 in the vast majority of specimens examined, is short in both sexes; 

 but in four males of small size from the vicinity of Penang, it is 

 elongated as in the other species. The specific identity of these 

 individuals is proved beyond doubt by the structure of the 

 petasma. It seems, therefore, that in A. erythraeus the males are 

 dimorphic; but it is very strange that no males with a long 

 peduncle occur among several hundreds of specimens from other 

 localities. 



The sexual characters of the female are more difficult to 

 observe, but the third thoracic sternite offers distinctive charac- 

 ters in each species; in A. japonicus its structure differs widely 

 from that of any of the allied forms. 



The species may also be distinguished by characters other 

 than sexual, and these, though for the most part less convenient, 

 are useful in the determination of females and young individuals.^ 

 Such characters are to be found in the proportions of the eye, in 

 the form of the second segment of the antennular peduncle, in the 

 ultimate segment of the third maxillipede, in the presence or ab- 

 sence of a tooth on the inner margin of the basis of the third 

 peraeopods, in the external border of the outer uropod and in the 

 telson . 



It is evident from the collection in the Indian Museum that 

 two species are often found together. In such cases, and when 

 the specimens are numerous, identification is a tedious process, 

 for each individual must be separately and carefully examined. 



Kishinouye was mistaken in supposing that the species can 

 be determined by the number of teeth on the rostrum ; in the 

 forms I have examined the rostrum is almost identical in struc- 

 ture. This is also true of the toothing and sculpture of the 

 carapace. The proportionate lengths of the segments of the third 

 maxilHpedes and legs afford only very slight differences, and I have 

 not been able to find any distinctions in the number or size of 

 the branchiae. 



The species of Acetes are usually found gregariously, swim- 

 ming in great numbers in mid-water or near the surface. They 

 are apparently only met with in coastal waters ; they occur near 



i The petasma is, however, sufficiently developed for accurate identification 

 i n specimens less than half the maximum length attained by the species. 



