224 WALTER GARSTANG. 



and the range of variability in the different species, are as yet 

 very inadequately determined. 1 would only remark that the 

 beaded ridge which bounds the posterior and postero-lateral 

 borders of the carapace, and which frequently bears a pair of 

 tubercles in its course (see fig. 2), is clearly the horaologue of 

 the posterior dentated ridges of Calappa and Hepatus, and 

 is probably degenerate in character. If this is true, the 

 variations presented by this ridge during the stages of its 

 disappearance are little likely to furnish the satisfactory 

 characters for specific discrimination which some systematists 

 have ascribed to them. 



3, Albunea symnista, microps, and scutelloides, n. sp. 



The problem of a pure water-supply in the case of sand-bur- 

 rowing crabs has been solved in certain instances, as I have 

 elsewhere shown (1896, 1897) in a manner even more original 

 than that which I have illustrated for Calappa and Matuta. 

 In the forms to which I refer (Corystes cassivelaunus, 

 Portumnus nasutus, and Atelecyclus heterodon of the 

 British coasts) the normal respiratory current of water — the 

 constancy of whose direction has been an accepted maxim among 

 naturalists since Milne-Edwards' classical elucidation of the 

 process nearly sixty years ago, — ths normal current is actually 

 reversed in direction, and flows through the branchial chamber 

 from before backwards, instead of from behind forwards. In 

 Corystes cassivelaunus 1 have shown that it enters the 

 chamber through a long tube formed by the apposition of the 

 second antennae, whose double rows of hairs interdigitate with 

 one another in a most effective manner (Plate 14, fig. 3). 



I now show that the structure of the first antennte in the 

 genus Albunea is strikingly similar to that of the second 

 antennae in Corystes, and that the tube formed by their 

 apposition has the same relations to the branchial chamber as 

 in Corystes. The species of Albunea are known to have 

 sand-burrowing habits of life, so that in all probability a 

 reversal of the branchial current takes place in this genus as 

 in Corystes. 



