CASH: FOSSIL MOLLUSCA. 
87 
Dibrancbiata and the Tetrabranchiata. 
The order Dibranchiata comprehendH two sub-orders. 
Octopoda and Decapoda. I think it possible to divide the 
Octopoda into Monocotylea and Polycotylea according* as the 
arms bear a sing-le row or several rows of suckers (2 in Octopus, 
3 in Tritaxeopus). The Decapoda form 3 large groups founded 
on the structure of the dorsal internal pen, which is Ciirtilaginous 
with the Chondrophora, Calcareous but without a chambered 
rostrum or g-uard in the Sepiophora (Ex. Sepia) and terminated by, 
or built up of, a series of air chambers, traversed by the siphon 
[U the Phragmophora (Belemnites, Spirula). The Chondrophora, 
which are the most numerous, may be re-divided after D'orbigny 
into Oigopsidge (Ommatostrephes) and Miopsidft? (Loligo). 
The order Ammonea is divided into Retrosiphonata and 
Prosiphonata, as the neck of the siphon is bent in a backward 
or in a forward direction. The Retrosiphonata have for the type 
the Goniatites ; the Prosiphonata includes all the shells called 
hitherto Ammonites. When these are unfurnished with an 
Aptychus, or possess only a horny Aptychus, they form the 
section Anaptychidea ; when their i^ptychus is made up of two 
calcareous pieces, free or soldered together, they belong to the 
section Aptychida. Fmally, the Anaptychida have been re- 
arranged by the shape of their first initial saddle into Latisellata 
and Angustisellata. 
The order of the Tetrabranchiata is divided into Prosiphonata 
and Retrosiphonata after the direction of the siphonal neck. The 
Retrosiphonata comprehends two families, the Nautilid^ with 
chambers perpendicular to the axis of the shell, and the Ascocera- 
tidge in which the chambers are very oblique, becoming even 
sub-parallel to the axis. 
Dr. Fischer's classification may be tabulated in the manner 
following : — 
