8 THE VOYAGE OF H.M.S. CHALLENGEE. 



2. Dactyle of raptorial claw not dilated at the base, but usually armed 

 with marginal spines. 



(1) Primary marginal spines of telson small, with no more than 



four secondary spines between the submedian and the inter- 

 mediate; outer spine of basal prolongation of uropod usually 

 longer than the inner ; dactyle of raptorial claw with not 

 less than six marginal spines. 



Genus Lysiosqiiilla (PL X. figs. 8-16). 



(2) Primary marginal spines of telson large, with more than four 



secondary spines between the intermediate and the sub- 

 median ; inner spine of basal prolongation of uropod longer 

 than outer ; dactyle of raptorial claw usually wdth no 

 more than six marginal spines. 



Genus Squilla (Pis. I., II., III.). 



In each of these genera there are certain characteristic or typical species, which are 

 sharply cut oflf from all other genera, but it is difficult to give any absolutely diagnostic 

 generic characteristics, as, in addition to the divergent and typical species, each genus 

 also contains a few species which are more primitive, with the characteristics of the 

 genus very slightly developed, and with features of resemblance to the primitive 

 species in other genera. 



The form of the dactyle of the raptorial claw affords a ready means for distinguishing 

 species, and most of the genera are based upon peculiarities of this organ, which furnishes 

 a tolerably satisfactory index of relationship, but gives no clue to the wider and more 

 deep-seated affinities; but as soon as we ignore the preponderating importance which has 

 been attached to the big claw, and take the whole organisation into consideration, we 

 find that there are, in each genus, species which exhibit evidences of relationship to a 

 common type or ancestral form, from which the various genera have diverged, and which 

 was characterised by the possession of small, subcylindrical eyes, an acutely pointed 

 rostrum, a smooth hind body, a short wide smooth carapace, very small antennary scales 

 and uropods, and a telson which was wider than long, with the marginal spines crowded 

 backwards, and the posterior border transverse, or nearly so. 



From this primitive form, which is represented at the present day, probably with 

 slight secondary modifications, by the various species which I have associated under the 

 generic name Protosquilla, the various genera have diverged, and while it is not at all 

 probable that any species which we know is the actual stem form of the order, yet there 

 is ample evidence to show that this was characterised by the features which the various 

 species of Protosquilla have in common, and that it must have been more nearly allied to 

 them than to any other species with which we are acquainted. Thus, for instance, the 



