272 
INFERIOR OOLITE AMMONITES. 
a form like Steplum. subcoronatum, or Brodiei, or perhaps of a more uncoiled form 
nearer to Stephan. Humphresianum. 
In the latter case the ventral furrow would have been acquired after the sides 
had become more convergent, and the whorls had lost the peculiar Blagdeni-&hape. 
What tends to confirm this is the fact that in several specimens of Stephanoceras 
a distinctly depressed band may be noticed down the middle of the abdomen almost 
cutting through the ribs.* 
These remarks concerning P. niortensis, which will be supplemented at a later 
date, will, I hope, be accepted as proving that Tmetoc. scissum is not the 
ancestor of P. niortensis. If Tmetoc. scissum were the ancestor, the inner whorls 
of Park, niortensis ought to show the form of Tmetoc. scissum — circular whorls, 
single ribs, and a sulcate abdomen. They disclose, instead, a coronate Ammonite, 
with divergent whorls, and with ribs crossing the abdomen — in fact. Am. pettos. 
The only further development of Tmetoc. scissum is into a more compressed, 
more involute form with closer ribs. This may be called normal development ; 
and that there was a tendency to produce this form is shown by the three 
specimens of Tmetoc. scissum figured. 
Two species which probably belong to this genus, " Park, clifalensis " (Gemm,), 
and " Park. Veneris " (Gemm.), have been referred to by Haug. They have not, 
however, as yet been figured or described to my knowledge — only mentioned in 
an abstract. Besides these he makes a new species " Sutneri " ^ of Dumortier's 
1 The pettos-Blagdeni "-form may be termed the " coronate radical." The similarity which exists 
between species sprung from this radical is due not to their being descended one from another, but 
to their being the homoplastic developments of this radical. The "coronate radical" threw off a 
series in the Upper Lias, namely, Dactylioceras, of which Z). crassum is the less specialised, while 
D. annulatum, &c., are the most senile forms. Waagen, Wiirtenberger, &c., have considered these 
— the Lias Planulati^^ — as the ancestors of PerispMnctes ; but this is a mistake. Perisphinctes 
is descended from a later " coronate radical," through certain forms from the Upper beds of the Inferior 
Oolite. The so-called ''^Perisphinctes " are polygenetic. Some are probably descendants of Parkin- 
sonia, for certain PerispMnctes show unmistakeable traces of a smooth abdominal band, and certain 
old ParhinsonicB have decided periodic constrictions ; while " Perisphy Davidsoni is apparently 
descended from Park. Garantiana. Other so-called " PerispMnctes " are probably senile develop- 
ments of the StepJianocerata of the Inferior Oolite, which again are not one group by any means, but 
are successive branches thrown off from the " coronate radical " — branches which passed through the 
same changes, — branches which were, in fact, homoplastic. Several tended towards greater evolution, 
and to produce a form li^e P . Martinsii ; while Steplian. zigzag when adult reassumes the gibbous 
whorls, only with greater involution, loses its distinctive ribbing, and produces Perispliinctes (J) 
procerus. 'Jlo '' zigzag such forms as " Herveyi,^'' " macrocepJiahis," " ariustigertis,^' &c., probably 
owe their origin. 
2 " Polymorphidse," ' Neues Jahrbuch fiir Mineral.,' &c., Bd. ii, p. 151, 1SS7. Haug relies on tbig 
form as being the intermediate between Amm. scissus and niortensis on account of its bifurcate ribs. 
The similarity is, of course, curious, but the fact to be got over is the shape of the inner whorls of 
Parkinsonia niortensis. 
