28 DR. G. C. BOURNE ON THE RANINID.E : 
second maxilL-e are narrow, tlie inner lobes normal (Ortmann). Finally, as 
I shall show, the nervous system is more Astacuran in type than in any 
otiier Brachyura. In addition, the Raninid* exhibit a number o£ features 
peculiar to themselves, and these I shall discuss in detail in the later part of 
this paper. 
For the present it is only necessary to refer to the theory originally pro- 
pounded by Huxley and brilliantly sustained by Bouvier in his essay, ' Sur 
I'origine homarienne des Crabes' (22), that the crabs are not descended from 
Paguridse nor from Galatheidas nor Thalassinidfej but directly from the 
Astacura, and in particular from the Homaridje through the Droniiacea. 
" Les Dromiaces par I'intermediare des Homolodromies ou de formes tres 
voisines, derivent directement des Hon)arid^s jurassiques ou de Jlomarides 
qui leur resseniblent." Exception being made of such crab-like forms as 
Porcellana, Lithodes, and the Porcellaiiopafiurus i-ecentlj' described by 
Borrodaile, Bouvier's theory of the origin of crabs is now generally accepted, 
and it follows that, if the Eaninidaj be true crabs, they must be descended 
from a Homarid or Nephropsid ancestor by way of the Dromiacea. Thus 
Bouvier (22) has given convincing arguments for the derivation of the 
Dorippidas from the Dromiacea, and Alcock (2) sees in his genus Cymonomops, 
which differs but little from Cydodorippe, a number of Raninoid characters 
which, he holds, warrant the conclusion that " \\ e have, in fact, in some of 
these deep-sea forms the clearest evidence of the close relation of the lianina 
to the Dorippe type and quite sufHcient justification for accepting de Haan's 
scheme of the Oxystomata, almost without modification.'' Cymonomops, 
however, is clearly a highly-specialised Gyclodorippid ; it has been placed in 
this family by Bouvier, and its supposed resemblances to the Kaninidffi are 
of the most superficial character. If the last-named family were descended 
from the Dromiacea it would hardly be through a Dorippid line, but it is mjr 
object to show in the following jsages that the Raninida?, though by defini- 
tion they must still be included among the crabs, are not derived from a 
Dromiid ancestor, but have been evolved as an independent group from the 
Astacura. In sustaining this thesis I shall place reliance first of all on the 
nervous system, and secondly on the character and degree of development 
of the endopliraginal skeleton. 
Our knowledge of the nervous system of the Decapoda is not very exten- 
sive, and what we do know is due largely to the researches of Bouvier (21). 
He has demonstrated that the degree of concentration of the nervous system 
varies so considerably in the several groups of Decapoda reptantia that it 
cannot of itself be taken as a sure guide of atfinity, but I shall be content 
to found my argument upon a sentence taken from his essay, ' Sur I'origine 
hoaiarienne des Crabes.' " Le systeuie nerveux des Crustaces deoapodes, en 
effet, subitune concentration longitudinale reguliere a mesure qu'on d'eloigne 
