44 PROCEEDINGS COTTESWOLD CLUB 1907 
General Characters:—Long loop; strong septum, 
about one-third of valve; dental plates; muscle-marks as 
in Pl. VI., fig. 19; small beak perforated by a minute 
foramen (when uninjured); strong beak ridges; very 
narrow areas each side of beak, so that the ventral valve 
extends but little beyond the dorsal; deltidial plates very 
short (I mm.) but very wide (7 mm. across in C. num- 
mosa), their angle about 112. : 
Remarks :-—The details of the beak and beak-area are 
the best characters for the separation of the genus Czucta 
from other genera. [The minute foramen in a small beak, 
and the narrow beak-area are both good features; but the 
wide-spread deltidial plates, when they can be seen, are a 
particularly useful feature of distinction. Thus the longer 
and narrower deltidial plates of swbmumzsmatis, with an 
angle of about 60°, shew at once that the species does not 
belong to the genus Czxzcta; but it shares this form of 
deltidial plates with Zez/erza, to which genus its other 
beak-characters assign it. 
The considerable amount of material of the genus Czxcla 
which has been examined has shewn that there is a large 
number of different forms, exhibiting much diversity of 
shape. To distinguish these forms it has been necessary ° 
to employ several names; but though the term “species” 
is used in connection with these names, it is not intended 
to give any definite opinion as to whether these forms are 
species, or subspecies, varieties, or mutations, or in some 
cases, perhaps, hybrids. The names are merely designa- 
tions for classificatory convenience; and by the use of 
a number of names in a strictly limited sense, it Will 
be found possible to accumulate information about each 
different form, so that, later on, the relationship of one to 
another can be stated with greater precision. 
In order to shew the affinities and differences of these 
various forms at a glance, the following Table may be 
even — 
