1909.] CRUSTACEA OF THE GENUS GENNADAS. 719 



was able to trace the derivation of the genus from the more 

 primitive Benthesicymus. This short paper was followed in 1908 * 

 by a lavishly illustrated memoir on the Penseida^ collected by the 

 Prince of Monaco, containing fuller treatment of the same species 

 in addition to valuable information on other genera. These two 

 papers have greatly facilitated any further work on the subject 

 and the revision of the ' Challenger ' material has in consequence 

 been robbed of much of its difficulty. 



Before going further it is, however, necessary to refer to the 

 generic status of the species belonging to this group. In 1882 t 

 S. I. Smith described, under the name of Amcdo2)enceus, a genus 

 which differs from Spence Bate's Gennadas only in the total 

 suppression of the podobranchs on the first three pairs of pereiopods. 

 For some time it was thought that Spence Bate's determination 

 of the branchial foi-mula was incorrect — a not unreasonal^le 

 hypothesis; Alcock, however, stated in 1901 that these gills were 

 present in certain specimens of Gennadas from Indian waters, 

 and an examination of the type species in the British Museum 

 establishes the coi^rectness of Spence Bate's observation. 



On the other hand, specimens of Avialojyenceus elegans from the 

 IST.E. coast of America show no trace of these gills, thus confirming 

 Smith's determination and that of several subsequent writers. 



The question now ai'ises whether the presence or absence of 

 these gills is of itself a factor of sufficient importance to justify 

 the retention of two distinct genera — for it is almost cei'tain that 

 no other characters are available for their sepai^ation. Although 

 the literature of the subject contains numerous references to 

 this question, Bouvier, strangely enough, makes no mention of it % 

 in his memoir on the material collected by the ' Princesse- 

 Alice.' 



The nomenclature of the species is pei-haps a matter of minor 

 importance, if the relationships of the various forms are fully 

 understood. Podobranchs ai-e rarely found on the thoi'acic limbs 

 of Decapoda Natantia ; they are most frequently present in the 

 Penseidea, and in such a -tribe, which abounds in jDrimitive 

 characters, the absence of these gills is rightly regarded as a 

 feature of great importance, for it indicates in no uncertain way 

 the degree of specialization to which the species has attained §. 

 I have consequently retained Amalopenceus as a distinct genus 

 and consider Bouvier's group, Benthesicymte, to comprise three 



* Res. Camp. Sci. Monaco, xxxiii. 1908. 



t Bull. Mus. Comp. Zool., Harvai-d, vol. x. 1882. 



X Bouvier {loc. cit., 1908) regards Amalopenceus as a synonj'm of Gennadas and, 

 in reference to the gill formula, merely states that it is the same as that of 

 Benthesicymus. This is certainly inaccurate for at least one of the species he was 

 dealing with, viz. Amalopenceus {Gennadas) elegans. 



§ Although the determination of the complete gill formula in these species is a 

 matter of some difficult}', the presence or absence of podobranchs on the first three 

 pereiopods can be observed with the greatest ease. The podobranch iu Gennadas 

 (PI. LXXIV. fig. 6) is an outgrowth from the base of the epipod; in Amalopenceus 

 a considerable space intervenes between the epipod and the lowest gill (an arthro- 

 branch). 



