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the Hexapodinae a subfamily of the Pinnotheridae, to a considerable widening of the present 

 group, iiicluding even such genera as Malacosoina and, though doubtfully, also Pinnotherelia 

 and Asthenognathns. Alcock, on the contrary, whose views are here foliowed, strictly adheres 

 to the absence or presence of the fourth pair of walking legs, allowance being only made for 

 AmorpJioptis. It is certainly undeniable that the existence of the Hexapodinae renders a sharp 

 demarcation between Goneplacidae and Pinnotheridae somewhat vague, and that they may be 

 referred with as much right to either of these families. 



It is most likely the commensalistic mode of life of the Hexapodinae that has brought 

 aboLit not only the cylindrical shape of the body, biit also the disappearance of the posterior 

 legs, which, by the fact that they are inserted at a higher level than the preceding pairs, 

 perhaps would rather impair the animal's moving up and down in the tubes of Annelids and 

 Hydrozoa. In many cases, however, the crabs are found outside their hosts or in indifferent 

 surroundings. Whether a tracé of the fourth pair of walking legs still exists in the very young 

 individuals is unknown, but a specimen of Hexaptis sexpes with a length of only 3 mm., 

 examined by A. Milne-Edwards, presented no vestige of degenerated posterior legs. 



Key to the genera : 



1 . Fourth pair of ambulatory legs present in the shape of a tubercle, 



at the base of the coxopodites of third pair Amorphopus Peil ^) 



Fourth pair of ambulatory legs completely absent 2 



2. Ambulatory legs very much elongate, slender, the meropodites of 



third (last) pair being \\_ times as long as the carapace. Front 

 narrow, much widening anteriorly; eyes very large, hammer- 



shaped. Deep sea species Hexaplax Doflein 



Ambulatory legs thick and short 3 



3. Propodus of external maxillipeds angular, broadened distaliy and 



attaining about the width of the narrow merus Thaumastoplax Miers 



Propodus of external maxillipeds of normal shape, cylindrical. . 4 



4. First sternal segment of cT with two deep, clearly-defined, trans- 



verse trenches, into which the distal part of the rectangularly-bent 



sexual appendages fits Lambdophallus Alcock 



Trenches in the first sternal segment of cT ill-defined and short, 



situated immediately beneath buccal cavity Hexapus de Haan. 



Thaumastoplax Miers. 

 r88i. Thaumastoplax Miers. Ann. Mag. Nat. Hist. (5), v. 8, p. 261. 



Were it not for the unusual shape of the external maxillipeds, this genus would certainly be 

 identical with Hexapus, but merus and ischium of these appendages are very narrow and weak. 



> 



i) Journ. Linn. Soc. London, v. 3, 1859, p. 27. The only species, A. cylindraaiis^ is, up to the present time, represented by 

 one single specimen, the habitat of which is not stated. 



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