200 A HISTOEY OF RECENT CRUSTACEA 



carapace projecting ; the eye-stalks obscure, ' immovably 

 lodged in an orbit excavated in the dorso-frontal margin 

 of the carapace, more or less covered by the antero-lateral 

 margin of the carapace ; ' the second antennae terminating 

 in a long and slender flagellum ; the first four pairs of 

 trunk-legs chelate ; the fifth pair simple in the male, some- 

 times chelate in the female ; the pleon not longer than the 

 carapace. The type-species, Polycheles typhlops, Heller, was 

 first taken in the Mediterranean. Since then various species 

 have been recorded from both the Atlantic and the Pacific, 

 and from depths varying from 220 to 1,070 fathoms. Since 

 in the female all the legs are usually chelate, the generic 

 name, meaning ' with many chelae,' is not inappropriate. 



Pentacheles, Spence Bate, 1878, meaning 'the creature 

 with five chelae,' seems to differ from Polycheles only in the 

 particular alluded to in the generic name, the male in this 

 instance, as well as the female, having the fifth pair of legs 

 chelate. The genus has a wide range in both the great 

 oceaDS, and the species descend to great depths. 



Spence Bate observes that Pentacheles euthrix (v. Wil- 

 lemoes Suhm) has a close general resemblance to his own 

 Polycheles laccata. 



Stereomastis, Spence Bate, 1888, is said to differ in 

 nothing externally from Pentacheles, but to be established 

 ' to receive those species in which the mastigobranchial 

 lash does not exist.' It was probably foreseen that some 

 apology would be expected for such a definition, and the 

 remark is accordingly appended, that ' difference of in- 

 ternal structure as a specific character is of more value 

 than any external distinction, which, though more con- 

 venient for classification, is of little importance if it does 

 not represent structural variation.' Yet the example of 

 the present genus gives but feeble support to this senten- 

 tious aphorism, especially as in the two preceding genera 

 the mastigobranchial lashes are for the most part of great 

 tenuity, and in Stereomastis Suhmi, Spence Bate, the third 

 maxillipeds have ' a rudimentary mastigobranchial plate,' 

 though the trunk-limbs are without any. The meaning 

 of the generic name would naturally imply the presence 



