PYCNOGONIDA— CALMAN. 9 



considerations phylogenetiques applicables a uu auti-c." I only refer, therefore, to one 

 case among fishes, to which Mr. C. Tate Regan has called my attention, where the 

 parallel seems unusually simple and complete. Until recently, the only Selachians 

 known to have more than five pairs of branchial arches were the Notidanoid sharks, 

 and as these are, in other respects, generalised and ancient types, the increased number 

 of arches may, not unreasonably, be regarded as a primitive character. Mr. Reo-an 

 (1906, p. 1), however, has described under the name Pliotrema a Pristiophorid shark 

 which has six arches. There can be no question that this is a very highly specialised 

 form, and that it has been derived from some form like Pristiophorus with the normal 

 number of branchial arches. The parallel between Pliotrema and Pentapycnon in their 

 relations to Pristiojjhorus and Pycnogonuin respectively seems to me very strikino-, and 

 it is hard to believe that arguments regarded as conclusive in one case can be without 

 value in the other. 



VII.-NOMENCLATURE AND TERMINOLOGY. 



In this report certain nomenclatorial changes suggested by recent authors have 

 been adopted, although they involve the rejection of long-established names or even 

 their transference in a manner against which I have elsewhere ineffectually protested. 

 They are adopted because they appear to comply with the only code of rules that 

 commands any general assent at the present time ; and because when once such 

 changes have been introduced in works of authority it is hopeless to try to prevent 

 their ultimate adoption. 



The terms used for the parts of the animal in the descriptions are, in the main, 

 those adopted by Prof. D'Arcy W. Thompson (1909) with some modifications that do 

 not call for special explanation. In the measurements, the " length of trunk " is taken 

 from the frontal margin of the head above the proboscis in the middle line to the base 

 of the abdomen, or the anterior margin of its socket if it is articulated ; the 

 " cephalon " is regarded as extending from the frontal margin to the base of the first 

 pair of lateral processes ; the " cephalic segment" is measured from the frontal margin 

 to the line of articulation between the first and second pairs of lateral processes. 



VIII. SYSTEMATIC NOTES AND DESCRIPTIONS OF 



NEW SPECIES. 



Genus COLOSSENDEIS, Jarzynsky. 



Mr. Hodgson has described, from the collections of the " Gauss," a species which 

 he makes the type of a new genus under the name of Notoendeis germanica. I have 

 not seen the type-specimen, but, to judge from the preliminary account, the genus 

 ^ould seem to be of doubtful validity. The only characters mentioned that are in any 



VOL. Ill, 



