Bibliographical Notices. 65 
flourishing, and then rapidly died away. From these facts Prof 
Blake considers that we obtain independent confirmation of laws 
which appear to widely govern the development of life, and may be 
thus stated :—“ The simpler forms of a class are the first to be in- 
troduced, and the more complex appear later. It is only when the 
class is in its most flourishing condition, and not long before the 
close of a period, that it throws out the more remarkable and ab- 
normal forms. The group which somewhat represents the mean of 
the whole, and never attains an extraordinary abundance, as the 
Sprrarzs, is the longest to last.” ‘The different forms of shells in 
the Nautiloidea of the Paleozoic rocks and their apparent successive 
development from the straight to the curved form, the reverse of 
which succession takes place in the Mesozoic Ammonitide (viz. 
from the curved to the straight form), more than thirty years ago 
arrested the attention of Von Buch, who made some ingenious 
suggestions as to the causes which brought about the reversed con- 
ditions in the two groups (Ann. & Mag. Nat. Hist. ser. 2, vol. v. 
1850, p. 382). After alluding to the various modifications which 
- take place in the Ammonitide until their final disappearance in the 
Cretaceous strata with the Baculites, Von Buch remarks “ that in 
the same manner as the Ammonitide vanish from the world, in the 
same manner exactly do the Nautilidz make their appearance in the 
oldest. The Ammonite vanishes through a series of forms between - 
it and the outstretched Baculite; the Nautilus, on the other hand, 
rises through a similar series of forms from the long-extended Ortho- 
ceratite.” 
it must not, however, be forgotten that, just as the straight Ortho- 
ceras continues contemporary with the successive appearance of the 
other forms in the Palsozoic rocks, so the curved Ammonite con- 
tinues throughout with all the other modifications of the Ammoni- 
tide. 
With regard to the geographical distribution of the British species 
as given in the table above referred to, 6 of the 143 species are 
found in America, and 32 are common to Europe; of these, 24 occur 
in the Lower Ludlow, and 18 in that and other strata. 
Prof. Blake next considers the character of some genera and their 
appearance in time as at present known. Cyrtoceras first appears 
in the Lower Tremadoc, followed in the Upper by the Orthoceras, 
the less simple form preceding the straight one; but, says the 
author, the history of discovery shows that we can place but little 
trust in such an isolated fact, which is liable any day to be reversed. 
“« Nevertheless, on any theory of evolution, this is just what we 
might expect; for the lower groups from which the Cephalopoda 
might be derived are not straight, like an Orthoceras, but curved, 
like a Cyrtoceras. Moreover we should expect, from the frequent 
curvature exhibited near the apex in the Orthocerata, that their an- 
cestors were curved.” 
Another fact, in relation to the position of the siphunele, is neticed, 
that the endogastric group antedates the exogastric in Cyrtoceras, as 
does the endogastric Phragmoceras that of the usually exogastric 
Ann. & Mag. N. Hist. Ser. 5. Vol. x. 5 
