PYCNOGONIDA -CALMAN. 
considerations phylogenetic{ues applicables a un autre.” I only refer, therefore, to one 
ease among fishes, to which Mr. G Tate Regan lias called my attention, where the 
parallel seems unusually simple and complete. Until recently, the only Selachians 
known to have more than live 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. Regan 
(1906, p. 1), however, has described under the name PUotrema 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 PUotrema and Pentapycnon in their 
relations to Pristiophorus and Pycnoyonum respectively seems to me very striking, 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 AY. 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 
would seem to be of doubtful validity. The only characters mentioned that are in any 
VOL. m. 
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