A STUDY IN' CARCINOLOGY. 45 
rareeau iiiEerieure ilii secoml aiineau, de facoii a entourer coinpletement le 
segment oculaire qn'on n'apeieoit pins qu'a rinterieur de la carapace " (25^ 
vol. i. p. 250). 
It is not necessaiy to push the question further, for the above quotations 
show that the terms " epistonie " and '" mouth-frame " (cadre buccal) have no 
definite connotation, and being incapable of exact application, must be dis- 
carded in favour of a more precise terminology founded on an appreciation 
of the morphological elements entering into the composition of this region. 
It is, however, no easy task to give such an appreciation. 
A consideration of the more simple and primitive arrangements obtaining 
in the JVIacrura must precede a discussion of the highly modified relations of 
homologous parts in the Brachyura,and for this purpose Nephrops norvegicus 
may be selected as an example. Fig. 19 is a frontal and fig. 20 a lateral 
view of the cephalic segments of this species. In both the rostrum has been 
cut through near its base, the ocular peduncles have been cut through near 
their origins, and in fig. 19 the edge of the carapace has been cut away 
where it overlaps the external boundary of the right antennary fossa. The 
first point for consideration is, what constitutes the anterior extremity of the 
cephalothorax ? Not, I think, the rostrum. Huxley (38) has given reasons 
for regarding the rostrum as a forward prolongation of the tiiird or antennary 
somite, and although, following Milne Edwards, he based his identification on 
the somewhat doubtful evidence afforded by the anterior head segments of 
adult Storaatopods, there is no reason to go so far afield to obtain evidence 
in support of this view. In the deep-sea Peiiseidaj, BentJiesicymiis altus and 
Uemijieneus spinidorsalis, the ocular and autennulary segments are sufficiently 
distinct and freely moveable on one another to afford abundant justification 
for it. It is also evident from an inspection of fig. 20, that in Nejjhrops the 
ocular segment lies in front of the base of the rostrum and has its proper 
tergum in the form of a median sclerite of cartilaginous consistency, little 
if at all calcified, but none the less definite in form and extent and clearly 
recognizable as a distinct element in the exoskeleton of this region. The 
side-walls or epimera of the ocular segment are thin and membranous, but 
the sternum is a distinct triangular piece, scarcely calcified and of much the 
same cartilaginous consistency as the iergum. It has been recognised by 
all previous authors. Immediately behind the ocular is the easily recogniz- 
able autennulary sternum, and to the right and left of it are the articular 
sockets of the antennules, separated by the arthrophragms ii/iii from the 
antennary sockets. The autennulary segment is completed, as has been 
shown by previous authors, by the aliform calcified plates which form the 
greater part of th€ walls of the shallow excavations in wdiich the oculir 
peduncles rest when turned outwards and represent the epimera of the 
antennnlar}- segment. I agree with Huxley (38, p. 157) in identifying a 
