275 
Distribution of Families in Time. 
Cambrian. 
Silurian. 
Devonian. 
Carboniferous. 
Trias. 
Lias. 
Oolite. 
Cretaceou.<». 
Eocene. 
Miocene. 
Pliocene. 
Recent. 
Argonautidjfi. 
X 
X 
Teuthidae. 
X 
X 
X 
X 
X 
X 
X 
SepiadfB. 
X 
X 
X 
X 
X 
X 
X 
Belemnitidoe, 
X 
X 
X 
Ammonitidoo. 
X 
X 
X 
X 
X 
X 
Nautilida}. 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
Orthoceratidre. 
X 
X 
X 
X 
X 
CLASSIFICATION. 
If we generalise the points of structure described in the 
foregoing pages, we shall find that the genera of Cephalo- 
poda may be arranged into natural groups, somewhat as fol- 
lows : — 
Class — Cephalopoda. 
Mollusca, having eight or more arms surrounding the 
mouth, which is situated at the anterior end of a cylindrical, 
oval, fleshy body ; a muscular mantle covered with pigment 
spots investing the body, and forming a ventral cavity, 
within which are situated two or four plume-like gills; a 
tubular organ (the funnel), and having the intestine with its 
flexure towards the nerve centres. 
Order A. Tetrabranchiata. — Animals havinc: four orills, 
numerous arms (exceeding ten), which bear no suckers ; a 
chambered sliell, which is external, and bears a siphuncle ; 
no ink-bag ; pedunculated eyes ; mandibles, with calcareous 
tips ; and funnel not a comj)lete tube. 
Family A. Ammonitidcc. — Animal unknown; shell 
