244 
THE GEOLOGIST. 
founded on teeth collected by M. d'Alberti from Tsebingen, and some 
by M. Bronn, from Schwenningen, in Wiirtemberg. In his notice 
of this species, Agassiz seems to have imbibed the idea, which Sir 
Philip Egerfcon has followed, of there being a great uniformity in the 
teeth of Hybodus, and upon these grounds excludes from the figures 
7 to 18 of Plate 24, which he had assigned to H. plicatilis, all but 
fig. 10 and 13 ; raising 7, 8, 11, 12, 14, 16, into the species H. Mougeoti, 
and fig. 17 and 18 into H. polycyphus, and fig. 9 and 15 into H. an- 
gustus. These three species, however, unless they have something 
more definite besides this untenable character of non-variation to 
depend upon, must hereafter be looked upon as very questionable, 
as the variation of the teeth in our Chalk specimen must render evi- 
dent to every unbiassed mind. 
The question of the geological age of H. plicatilis, must rest on 
the correctness of the assignment of the so-called Muschelkalk beds 
of Taebingen and Schwenningen ; but there are, it will be seen further 
on, other specimens in our National Collection from beds termed 
Muschelkalk in other Continental localities : and from which, too, 
J£. ohliquus, H. rugosus, and H. suhJcevis are recorded. 
Agassiz is certainly wrong in admitting S. sulcatus at the recent 
end of the geological series as a Chalk specimen, and it would be well 
therefore to examine the right to the antiquity of those species he and 
others put at the older end of that series. I am not suiEciently ac- 
quainted with the Muschelkalk strata to do this myself. 
Still, so nearly allied as is the genus Acrodus, we may expect, if we 
believe in development doctrines, a few early preceding forms in an 
era older than the Jurassic — tho period of " reign," par excellence, in 
quantity of individuals and in number of species of the genus Hy- 
bodus. The teeth of our Chalk species — the most recent known — are 
more like those of the Muschelkalk species {H. plicatilis) than any 
other, and a comparison with fig. 1 of Agassiz' Plate 22a may well be 
made. This resemblance is not, perhaps, a little singular ; and we have 
a floating idea — not yet properly worked out by facts — that during 
the period of extinction of a genus or family, there is a tendency to 
sportive (?) varieties, and amongst these a greater or less disposition 
to revert to the originating form. For example, in the Cretaceous era, 
when the Ammonites died out, we see numbers of forms, Hamites, 
Crioceras, etc., reverting, as it were, back to the primitive Orthoceras, 
until in the Baculite of the Upper Chalk, where the chambered-cepha- 
lopods — excepting the Nautilus — cease altogether, we have a form 
