MODIi'lCATlONS OF STEUCTURE IN DECAPOD CRUSTACEA. 223 



tinuous process of adaptation to a sand-burrowing existence 

 The marginal dcnticulations becoming useless^ gradually lost 

 their sharp and prominent form, until they assumed the blunt, 

 irregular, variable and ol)solescent character which they ex- 

 hibit in the modern species of Matuta. The great epibran- 

 chial spine is in itself evidence of the validity of this view, for 

 it clearly represents the posterior spine of the antero-lateral 

 series in such genera as Bathynectes, Callinectes, and 

 Lupa. The same spine is again met with in the allied but 

 less specialised genus Mursia; and a comparison of M. 

 armata (De Haan, 1833, pi. xix, fig. 2) with M. cristata 

 (Milne-Edwards, ^ Regne Animal,' pi. xiii, fig. 1) confirms the 

 views I have put forward. In M. armata the epibranehial 

 spine is longer, while the antero-lateral teeth are absent ; in 

 M. cristata both are present, but the epibranehial spine is 

 less elaborately specialised and still forms part of the marginal 

 series. 



The reason for the great elaboration of this epibranehial 

 spine in Mursia armata and Matuta is less clear, and can 

 scarcely be found without special study of the living animal. 

 In its initial stage, however, as presented in Bathynectes 

 longipes, I have every reason to believe it functions as 

 a stay or barrier to the cheliped during apposition to the 

 carapace, thus mechanically maintaining the arm of the cheliped 

 in the right position for the closure of the exostegal canal 

 (1897, p. 400). It seems to discharge this function also in 

 Matuta picta, but I am doubtful wliether this function is 

 the only one which it discharges in cases where it is so highly 

 developed. 1 



I make no suggestions as to the utility of specific characters 

 in this genus, owing to the fact that the species of Matuta, 



' Krauss (1843) remarks on the frequency of similar spines in arenicolous 

 animals of various groups, e. g. fishes and molluscs, as well as crabs. One 

 function may be to protect the crab from the danger of forcible dislodgment 

 from the sand by wave-currents, as ably maintained by my friend Mr. Hunt in 

 the case of the spiny species of Cardium (' Proc. Linn. Soc.,' xviii, " Zool.," 

 p. 269). 



VOL. 40j PART 2. — NEW SER. R 



