■ 10 THE VOYAGE OF H.M.S. CHALLENGER. 



This ancestral form must have been very similar to two closely related living species, 

 Squilla hradyi, Milne-Edwards, and Gonodactylus ti-achurus, Miers, which are referred 

 by Miers to the genus Gonodactylus on account of the enlargement of the base of the 

 dactyle, and to Squilla by Milne-Edwards on account of the presence on the same 

 organ of marginal spines, and the flatness of the hind body. Their points of 

 resemblance to Gonodactylus are also points of resemblance to Protosquilla, and as they 

 differ from all the species of Gonodactylus in the flatness of the hind body, and the 

 presence of spines on the dactyle, there can be no doubt of the propriety of placing 

 them in a distinct genus, for which I propose the name Coronida. Coronida has, like 

 the convergent species of Lysiosquilla and Squilla, small eyes, antennary scales and 

 uropods, a fiat hind bod}^, an armed dactyle, and a wide rounded telson, and there can 

 therefore be no doubt of its close relationship to the ancestral type of these genera. 



The species of Coronida, Pseudosquilla and Gonodactylus, are closely related, but not 

 in such a way as to indicate that any one genus is the ancestor of the others. The two 

 latter resemble each other, and diflFer from the first, in the fact that the hind body is 

 convex and narrow and l)ent do'miwards at the tip, while it is straight and flat and wide 

 in Coronida. 



Gonodactylus and Coronida resemble each other and diS"er from Pseudosquilla in the 

 presence of an enlargement at the base of the dactyle of the raptorial claw, while 

 Pseudosquilla and Coronida resemble each other and difi'er from Gonodactylus in the 

 presence of marginal spines on the dactyle. 



This triangular relationship can be accounted for only on the hypothesis that they are 

 the divergent descendants of an ancestral form which each one of them resembles in 

 certain features, to which, in each genus, secondary difi"erences have been added. 



As there is no reason to suppose that this divergence is recent, we should not expect 

 to find this ancestral form still represented by living species, but as the living species of 

 Protosquilla exhibit, like this hypothetical stem form, features of resemblance to each of 

 these genera, it is not only probable, but almost certain, that they are much more directly 

 descended from the ancestral form than any of the species of Gonodactylus or Pseudo- 

 sqxiilla or Coronida ; and, of course, than any of the Lysiosquillse or Squills. 



Protosquilla resembles Gonodactylus in the small size and flatness of its carapace, in 

 the presence of an acute spine on the rostrum, in the unarmed dactyle dilated at its base, 

 in the height of the narrow hind body, the terminal somites of which are bent down- 

 wards, and also in the small size of most of the species. 



It also resembles Pseudosquilla in most of these features, but the dactyle of 

 Pseudosquilla is without the basal enlargement, and is usually armed, like that of Coronida 

 and the Squillae and Lysiosquillee. 



Coronida resembles Protosquilla in the small size of its ej-es, antennary scales and 

 uropods, the flatness of the small carapace, the enlargement of the base of the dactyle, 



