372 Mr. C. Chilton on Polymorphism 



in all these species the first pair are the same in the male as in 

 the female. In the forms under consideration exactly the 

 reverse is the case : the male forms differ from the female in 

 the structm'e of the Jirst pair of gnathopoda, while in Aora 

 typica^ Microdeuteropus Mortoni, M. tenuipes, M. maculatus ^ , 

 and M. maculatus $ , the second pair of gnathopoda is essen- 

 tially the same throughout; and the following description 

 drawn up from Lyttelton specimens of M. maculatus ? will 

 apply, with the slight variations afterwards mentioned, to 

 the second gnathopoda of all the other forms. 



The meros (see PI. X. fig. 5) is rather pointed distally and 

 has the distal margin supplied with a row of long setge ; the 

 carpus is triangular, the inferior or posterior edge rounded and 

 abundantly supplied with setse arranged in short transverse 

 rows, the setge being longest towards the distal end ; there 

 is also on the side of the carpus, near its distal end, an oblique 

 row of about six long setse, and more proximally a much 

 shorter row of from two to three setse ; on the upper or anterior 

 margin are two or three small tufts, the last being situated at 

 the antero-distal angle 5 the propodos is of the same width as 

 the carpus and has the palm slightly convex, transverse, or 

 but slightly oblique, very minutely serrated, and with a some- 

 what irregular fringe of rather short setse, and defined by a 

 stout seta ; the posterior edge of the propodos is thickly sup- 

 plied with setse arranged in about six short transverse rows or 

 tufts ; on the anterior edge are five similar rows, each con- 

 taining about five seta?, the last one being at the base of the 

 dactylos, and on the side of the dactylos are four or five 

 similar short transverse rows of setee, arranged in one longi- 

 tudinal line ; the dactylos is fairly stout, inner edge finely 

 serrated, the serrations increasing in size until near the extre- 

 mity, which is, however, smooth. All the set^ on the poste- 

 rior margin of the meros, carpus, and propodos appear to be 

 serrated throughout the distal half of their length, and many 

 are slightly bent in the centre, while those on the palm and 

 on the anterior margin of the various joints are simple. 



In the specimens of Microdeuteropus maculatus $ which 

 Mr. Thomson sent me the second gnathopoda agreed very 

 closely with the description given above, but had the palm 

 rather more transverse than in my Lyttelton specimens. Aora 

 typica, both Lyttelton and Dunedin specimens of which were 

 examined, also agrees with this description, but has the palm 

 more oblique, and generally has a greater number of setee 

 than in Microdeuteropus maculatus $ , though they are 

 arranged in precisely the same manner. 



The first gnathopod (see PI. X. fig. 6) of Microdeuteropus 



