302 DISCOVERY REPORTS 



telson of " A type ". There is no transition between it and the telson of " C. II "^ type, 

 but every gradation between it and " C. I" type, both in shape and in the position of 

 the lateral spines. It is found in its typical form in all the species of Group I, except 

 perhaps in B.R. II which is transitional. In Group II (without epipods) it is seen in 

 all species which have an exopod on leg 5 and also in those with four pairs of pleopods ; 

 but it gives place to a telson of C. I type in all those with three pairs of pleopods and no 

 exopod on leg 5 (Group II B) with one exception— B.R. VI. Species D. XIII may be 

 another exception, but it is not fully developed and may actually be the young stage 

 of D. X. 



(5) Abdominal spines. There is so great a variety in dorsal armature that no definite 

 deductions can be drawn therefrom. If Axius and Calocaris are typical of the Axiidae 

 we should expect Axiid larvae to have no dorsal spines on somites 2-5, or that they 

 should be small and perhaps paired. As between them and Callianassa type II, in which 

 these spines may be reduced or absent, no difference can be found. In Callianassa of 

 type I, however, the dorsal spine of somite 2 is very large and hollowed, and somites 3-5 

 have a pronounced ridge which may be serrated. These characters are found in all the 

 species of Group II B, which may safely be referred to Callianassa, but they are also 

 repeated in species of Group II A, with four pairs of pleopods and exopod on leg 5, 

 which certainly cannot be Callianassa. 



(6) In the following descriptions I have noted the presence in most species of a seta 

 placed either on the edge or on the ventral surface of the antennal scale near its base. 

 This seta may be long and plumose, or it may be reduced to a minute hair and so 

 difficult to see that it may sometimes have been overlooked. It is found in Axius 

 stirhynchus and in A. plectorhynchus, but not in Callianassa subterranea. Its occurrence 

 in larvae in each of the groups into which I have divided them shows that it cannot 

 be used to distinguish between Axiidae and Callianassidae ; it may in fact be evidence of 

 the common relationship of all, since it is not found, so far as I know, in any other 

 Thalassinidea. 



Having regard to the difficulty in applying any of these characters I have refrained 

 from attempting to assign any of these larvae to their true position; but I think it 

 probable that the whole of Group I belong to the Axiidae and Groups II B and C to 

 the Callianassidae. Group II A remains quite uncertain. It must be remembered that 

 the genus Callianidea is not yet accounted for, and it may well be that it has a larva 

 transitional between Axiidae and Callianassidae. 



Since the larval genus Oodeopus Bate covers the whole range of Axiid-Callianassid 

 larvae it might be a convenience to make use of this name ; but I have preferred to use 

 no names and to designate the larvae simply with a letter and number, the former 

 indicating their provenance — Discovery or Barrier Reef. 



Before proceeding to deal with the unnamed Axiid-Callianassid larvae I give here 

 a description of the unhatched larva of Axitis plectorhynchus, since it is important to 

 add to our knowledge of the development of named species of the group. 



^ For description of " C. I " and " C. II " types of telson, see p. 300. 



