TMETOCERAS. 
271 
becoming specialised, and is not a character inherited from an ancestor so remote 
as Tmetoc. scissum ; while the lateral spines, for the existence of which at the point 
of bifm-cation there may be said to have been some reason, have really lost their 
significance on the single rib, where, however, they are retained by inheritance, 
thus becoming vestiges of a former condition. 
It might be objected that the single rib of Park, niortensis is a reversionary 
character, pointing to, and derived from, Tmetoc. scissum.. Setting aside the lateral 
spines, which have certainly not been in any way derived from Tmetoc. scissum, 
this idea of the single rib being a reversion is fully disproved by the young of 
Park, niortensis. They show that the bifurcate ribbing was persistent in youth, and 
moreover that the inner whorls of Park, niortensis at a certain age are practically 
Park. Garantiana in miniature. By further breaking up a specimen of Park, 
niortensis it is seen that the ventral area was flatter, the sides more divergent, the 
ventral sulcus a mere shallow depression, and that the ribs faintly crossed the 
abdomen. Going still further back, it is seen that the ribs crossed the abdomen 
without any trace of a ventral sulcus or depression, and that the inner whorls 
belong to the coronate type — they are in shape exactly like a miniature Am. 
Blagdeni or Am. pettos. 
The inner whorls of ParZ;. niortensis thus, indicate these facts, at least — that the 
species is descended from a divergent-sided coronate Ammonite, with broad, flat 
abdomen, an Ammonite altogether like " pettos-Blagdeni," — that the ventral furrow 
is a later acquirement, — that until about half -grown it is almost exactly like Park. 
Garantiana, — and that the single ribs are the latest acquired character. Park, 
niortensis is merely a mutation of Park. Garantiana, and is specialising the single 
ribs and large ventral spines ; and, further, Parkinsonia itself is simply a 
development of Stephanoceras in which a ventral sulcus has been specialised out 
of the ventral interruption of the ribs seen in some species of Stephano- 
ceras. 
I must reserve more elaborate detail until I treat of the genus Parkinsonia ; 
but I may state that the broken up specimens of Park. Parkinsoni, Garan- 
tiana, and Gaumonti, some of which I have reduced almost to the birth- 
chamber, give the same results as Park, niortensis, and confirm the fact that 
the pettos-Blagdeni^^ -shsiipe is the radical form from which all the Parkinsonia 
commenced. 
Now, the " 'pettos-Blagdeni "-form is the radical of Stephanoceras — it is the form 
invariably found in the inner whorls ; while sometimes it persists throughout life 
as in Stephan. Blagdeni. However, Parkinsonia and Stephanoceras are sprung 
from the same source ; and it is more than probable — though the diSiculty which 
attends the breaking of Oolite specimens owing to crystallisation, &c., does not 
allow me to confirm the statement positively — that Parkinsonia is a descendant of 
