VOL. XVI. (1) BUCKMAN—GENUS C/NCTA 59 
expansion, making a flabellate form (Flabellate stage— 
adolescent C. cor) Then followed, in turn, development 
of bilobation and of thickened margin—shewn in the forms 
figured by Deslongchamps; but these forms are a kind of 
side issue of the C. cor-series. It must be from the 
flabellate pre-bilobate stage that the Pliensbachian Czacte 
arose. The flabellate form is followed by attempts to- 
wards renewed elongation; and as the flabellate form 
is inherited earlier and earlier—the principle of tachy- 
genesis—the greatest width appears towards the posterior 
part of the shell, after which there is more or less decrease. 
The greatest development in transverse shape is C. fer 
nummus, which may be said to have obtained an obflabellate 
form (reversed fan-shape.) 
In the Pliensbachian Czxcte, bilobation does not follow 
directly on the flabellate stage as it does in: Cacae <) en 
them there is first a return to elongation—post-flabellate 
elongate stage. In C. fernumemus this diminution of 
width has made little progress before signs of bilobation 
are seen; but in C. orbeculares there is considerable 
elongation without any sign of bilobation. In C. guadrans 
diminution of width is rapid: this gives the shell its 
peculiar shape. In C. paupercuia the renewed elongate 
stage must evidently affect the transverse stage, SO that 
transverse expansion is reduced, and return to the primi- 
tive elongate stage is marked. In C. meisera there is 
complete return to the supposed primitive elongate stage. 
Bilobation and marginal thickening are old age charac- 
ters which are developed independently. Thus the 
bilobation of C. xwmdsmadis is not inherited from C. cor 
directly. In C. uumzsmales there is bilobation without 
valve-thickening ; but in C. odo/s some marginal thickening 
precedes bilobation. So in C. opulenta there is marginal 
thickening before incipient bilobation , and in C. pingues 
there is marginal thickening without bilobation. 
