INTRODUCTIOX. 
IX 
of these coincides with the Ammonoidea, which are thus sepa- 
rated from the Tetrabranchiata on the ground that they form a 
natural division of the Cephalopoda, which, while they present 
some characters common to the Dibranchs (spherical first chamber 
&c.), nevertheless possess others which ally them with the Tetra- 
branchs. The Tetrabranchiata are divided into two groups : — (n) 
the Prosiphonata (necks of the septa directed forwards), containing 
the Family Xothoceratidae, represented by two genera, JS'othoceras 
and Bathmoceras ^Conoceras^^ ; {h) the Eetrosiphonata (necks of the 
septa directed backwards), comprising the two Families Xautilidae 
and Ascoceratidae, the former answering to the order Xautiloidea, 
Xothoceratidae and Ascoceratidae being excluded. The reason given 
by Fischer for including so many diverse forms in the same Family 
(Xautilidae) is that the construction of the aperture (simple or 
complex) cannot be employed as a Family character, on the ground 
that every structural type, whether straight, arcuate, discoid, or 
helicoid, possesses forms which exhibit both kinds of aperture ; and 
it is for this reason that he rejects the Family Gomphoceratidae 
established by Pictet for Xautiloids with complex apertures. It is 
not, however, upon the character of the aperture alone, important as 
that is, that Families are constituted, but upon a combination of 
structural features — septation, peculiarities of siphuncle, coiling 
or uncoiling of the shell, &c. 
Dr. Fischer has failed to recognize the close relationship between 
Actinoceras and Huronia, and the very characteristic and remark- 
able internal structure of the former, so amply illustrated by 
Bigsby\ Stokes^, Saemann^, and Barrande k AVe still find also 
(p. 410) Salter's figure of the siphuncle of Piloceras as the sole 
illustration of that genus, though Billings® long ago figured and 
described the siphuncle of Piloceras Wortlieni, with the septa 
attached. Such imperfections as these, however, detract very little 
from the general excellence of Dr. Fischer’s work, which is indis- 
putable. 
It was held by von Jheriug (as quoted by authors) that Tenta- 
culites was the prototype of the Cephalopoda. This view is sup- 
ported with much confidence by Professor Hyatt® upon embryonic 
^ Trans. Geol. Soc. ser. 2, vol. i. 1824. ^ Pid. vol. v. 1840. 
® Palseontographica, Band iii. 1854. 
* Syst. Sil. de la Boheme, vol. ii. 1866 ; plates ccxxxiv., ccxxxv., ccxxxvii. 
* Palaeozoic Fossils, vol. i. 1865, p. 256, fig. 240. 
® Proc. Araer. Assoc. Adv. Sci. vol. xxxii. 1883, p. 327. 
