266 Proceedings of the Royal Society of Edinburgh. [Sess. 
To appreciate the relation between these two types of structural 
conformation it is necessary to take a wider survey of the Idoteidse. 
Considering for a moment Richardson’s (1905) classification of the large 
number of forms occurring in the seas round the continent of North 
America, let us arrange her genera according to this single criterion, 
thus : — 
(1) All seven epimera separated — Erichsonella (3 species); 
(2) The last six epimera separated, the first completely fused — Mesidotea 
(2 species), Chiridotea (2 species), Idotea (8 species), Pentidotea (4 species), 
Gleantis (3 species) ; 
(3) The last three separated, the preceding four fused — Colidotea 
(1 species); 
(4) All seven epimera fused — Synidotea (14 species), Edotea (3 species). 
Premising, by way of parenthesis, that one of her generic types — Eusym- 
merus — based upon a single specimen, and therefore unreliable, has been 
omitted from the above list, one remarks that the serial order here 
exhibited is known on purely morphological grounds to represent the 
order of evolution of fusion. 
Type (1), represented by Erichsonella among the Idoteidse, is rare 
among isopods, so rare that Caiman (1909, p. 203) states: “In all Isopoda, 
with the single exception of the genus Plakarthrium (Sphseromidse), the 
coxopodites of the second thoracic somite (the first free somite) are 
completely coalesced with the body.” Type (3), w T hich would include 
Glyptonotus and Symmins, Richardson (1904), is less uncommon. Types 
(2) and (4) are the most prevalent. 
One peculiar feature about the series is that progression from one type 
to another is not continuous, but step-like. Between (2) and (3) there is 
a sudden jump, and an equally sudden jump between (3) and (4). Arguing 
too from the number of representatives in each group, we should say that 
types (2) and (4) appear in each case to represent a more stable set of 
conditions than either type (1) or type (3). 
In looking for some explanation of the anomaly we inevitably think of 
the moulting process in isopods — the reader is here referred to a previous 
communication, Tait (1917, II). So soon as we correlate the step-like 
progression with the stages and phenomena observed in the moult, we 
begin to realise how important it is, in attempting to frame proper con- 
ceptions with regard to criteria of classification, to study not the dead but 
the living animals, to compare not exclusively structure with structure 
but structure with questions pertaining to function. From this point of 
view too the occurrence of a specimen like “ Eusymmerusd which had 
