270 
INFERIOR OOLITE AMMONITES. 
" Am. scissus " (Benecke). Vacek's contention that " Amm. scissus " and ^'Dumor- 
tieri " are very closely connected is admitted by Haug ;^ and there can be little 
doubt that " ^TO. scissus" has developed from Pol. Zitteli (Haug) by gradually 
making more and more pronounced the nascent interruption to the ventral 
ribs — of " Zitteli " — until it has produced the deep furrow. 
That " Am. scissus,'''' however, is the ancestor of the Parhinsonise, or is in any 
way connected with them, as has been contended, not only by the authors 
mentioned but by many others, is a great mistake. This error arose from the 
similar appearance of "Am. scissus" and " niortensis " " — in all probability the 
ventral sulcation was the deceiving feature. It only shows that not the least 
reliance can, in genealogical matters, be placed on similarity of external ornaments. 
Such similarity is well described by Prof. Hyatt as " homoplastic, and not 
homogenous." ^ 
I place "Am. scissus" in a genus Tmetoceras (see p. 267), and will state my 
reasons for deposing it from the position of progenitor to the Parhinsonisd. In the 
first place, though both species agree in having a sulcate abdomen margined with 
spinous-ended ribs, yet Park, niortensis differs from Tmetoc. scissum particularly in 
one especial feature, namely, that it possesses a spine on about the middle of 
every rib ; but no such a spine is to be seen in Tmetoc. scissum on the lateral area. 
Next, in a series of Park, niortensis (d'Orbigny) it will be seen that the single rib, 
which is the rule in Tmetoc. scissum, is by no means so in Park, niortensis — in fact, 
bifurcate ribs, with their point of origin in the above-mentioned lateral spine, are 
by no means uncommon in Parle, niortensis ; and the smaller the specimen the 
more numerous are the bifurcate ribs. Really the single rib is a feature only of 
the adults, and of about a dozen nearly adult specimens of Park, niortensis now 
before me, not one but discloses occasional bifurcate ribs. From these facts it is 
safe to draw the following conclusions. 
The single rib of Park, niortensis is a lately acquired character which is 
1 ' Op. cit.,' Bd. ii, p. 149, 1887. 
2 I am rather surprised that Prof. Hyatt, whose comprehension of Ammonite developments is so 
remarkably exact, should fall into an error similar to this when he considers Am. Taylori as a 
Cosmoceras (Genesis of Arietidfe, ' Smithsonian Contrib. Knowledge,' 673, p. 23). 
^ The more Ammonites are studied, the more it will be seen that they are all descended from 
certain little-specialised species which Hyatt aptly terms " radicals." These radicals threw off, as it 
were, successive Ammonite branches, which became highly-specialised, and then died out. In 
becoming highly-specialised each branch followed with but little alteration — -less alteration than a 
superficial observer might imagine — the same cycles of changes ; and each branch produced successive 
and practically similar series of ornaments. These ornaments, however, differ in intensity, propor- 
tion, arrangement, duration, and small details ; but what really create the greatest apparent diversity 
among similarly-ornamented Ammonites are the very different shapes and the amount of involutiou 
which the whorl possesses. 
