CEUSTACEANS FEOM THE TEOPICAL ATLA^^TIC. 351 



are provided with a couple of serrate coupling-hooks on each peduncle, and a single 

 cleft spine on the first joint of the inner ramus. At least iu general there are fewer 

 joints to the inner than to the outer ramus. 



The species belonging to this genus are now numerous. Those earliest described 

 remain, and will probably for ever remain, involved in much obscurity. Astacus 

 crassicornis, Fabricius, 1775, Tyro cornigera, Milne-Edwards, 1840, and Tijro sarsi, 

 Bovallius, 1885, all with large upper antennse, may very likely be one and the same 

 species, which I am disposed to call Scina cornigera, rejecting the earlier crassicornis 

 on the ground of the too uncertain identity. The 8cma ensicorne, Prestandrea, 1833, 

 from the Mediterranean, may yet be identified, but the Clydonia gracilis and Clydonia 

 longipes of Dana are so figured that, in the absence of the type specimens and with 

 our present knowledge of the genus, I do not think they will ever be reconciled with 

 any actual species. Professor Chun recognizes that his Scina lejpisma stands near to 

 the Mediterranean Scina marginata of Bovallius, but considers it distinct because the 

 upper antennse are shorter, with strikingly strong armature of filaments, and because 

 it has four pairs of branchial vesicles instead of six. There does not, however, appear 

 to be really any difference in the armature of the antennse, and the difference in length 

 is not by any means considerable. As for the branchial vesicles, in Sciaa marginata 

 these are said to be found on all the limbs of the perseon from the second to the seventh, 

 but in Sci7ia lepisma only from the fourth to the seventh. This would constitute a very 

 important distinction, but the fact stands in need of confirmation. These vesicles are 

 very easily detached and lost in the handling of a specimen, and there is a great 

 improbability that a species should be without them on the second and third pairs ot 

 limbs, and yet have them on the seventh pair. Thus the validity of the species leinsma 

 is left in some doubt. Of his other species, Scina hovallii. Professor Chun says that it 

 has four pairs of branchial vesicles, " between the third to the seventh pairs of trunk- 

 limbs ; " and this he considers one of its chief distinctions from Scina horealis, Sars, and 

 Scina clausi (Bovallius), both of which have the normal set of branchial vesicles 

 extending from the second gnathopods to the fourth perseopods. Sars is rather 

 inclined to regard Scina clausi as a synonym of Scina borealis, but both clausi and 

 hovallii may be distinguished from borealis by different proportions in some of the 

 limbs. 



Arithmetic shows that a pack of fifty-two cards may be dealt out in a bewildering 

 number of ways. It may be noticed, therefore, that in the genus Scina the animals 

 have on the perseon seven pairs of limbs, each limb having six free joints, and 

 that they have also a pair of antennae consisting of peduncle and flagellum, and 

 three pairs of uropods, each uropod having one branch free from the peduncle. 

 Thus there are fifty-two pieces to be played with, each of which may be relatively 

 long or short, broad or narrow, simple or variously armed. Relatively also to each 

 other these eleven pairs of appendages may go through any number of variations 



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