64 Bibliographical Notices. 
lati 20, Angulati 6, Lineati 13, Imbricat: 7, and Leeves 22, the ex- 
ternal surface of the latter not being fully known ; then follow the 
subgenera, Actinoceras 1, Endoceras 3, Tretoceras 1, Conoceras 1. 
Orthoceras has its earliest representative in the Upper Tremadoe, 
attained its maximum in the Upper Silurian, and is well repre- 
sented in subsequent periods to the Trias. The 23 species of Cyr- 
toceras are divided, according to the position of the siphuncle, into 
Endogastric and Exogastric (adopted from Barrande); and Prof. 
Blake proposes a third division, Mediogastric (Mesogastric ?), with 
the siphuncle near the centre. 
Cyrtoceras commences in the Lower Tremadoc, and is represented 
in the Silurian, Devonian, and Carboniferous; the subgenus Piloceras 
is of Lower Silurian and Trigonoceras of Carboniferous age. 
Of the Inriatt, Gomphoceras has 11 and Phragmoceras 7 species, 
all of which are chiefly Upper Silurian ; the 3 speciesof the singular 
genus Ascoceras from the Ludlow rocks are fully described. 
The Sprrates are represented by 3 species of Nautilus from the 
Upper Silurian, and 3 of the subgenus Trocholites, of Lower Silurian 
(Bala) age. 
The last group, InrecuLarEs, includes T’rochoceras, 12 species, of 
which 8 are Upper and 4 Lower Silurian forms; Lituites has 2 
species from the Lower Ludlow. Litwites articulatus, Sow., and 
a new species are placed by Mr. Blake under Ophidioceras of Bar- 
rande, which differs from Jitwites in having the walls in contact ; 
the genus is only known in the Upper Silurian of England and 
Bohemia. 
The Silurian Cephalopoda range in time from the Tremadoc beds 
to the Upper Ludlow tilestones, as shown in the table, pp. 233- 
936. Of the 143 species the greatest number (65) occur in the 
Lower Ludlow, 48 in the Wenlock shale; and an equal number (39) 
in the Bala beds and Upper Ludlow, while the Wenlock Limestone 
contains 35 species. In the second (condensed) table, p. 237, showing 
the growth, culmination, and, in some cases, the decay of the various 
genera or groups, and thus giving some insight into the laws which 
govern the appearance and disappearance of forms of life, it will be 
observed that the larger number belong to the Conrcr group, which 
first appear in the Lower Silurian and contain the bulk of Lower 
Silurian forms ; but, in relation to the maxima of species, the Bala 
beds of the Lower and the Wenlock shale and Lower Ludlow of the 
Upper Silurian contain the greatest number, while, from the fact 
that the species in the Wenlock Limestone are fewer than in the 
shales either above or below, Prof. Blake infers “that the Cepha- 
lopods of those days were not commonly frequenters of clear and 
shallow waters, but were partly pelagic, and not uncommonly gre- 
garious in more or less turbid waters.” The Conicr and Sprratzs 
continue to flourish in later periods, the Nautilus of the latter being 
now the only living representative. » 
The two other more or less abnormal groups, Inriatr and IrRre- 
GULARES, although represented in the Bala beds, attained their maxi- 
mum in the Ludlow period, when the whole class was most 
