JL 2 . //^/ 
NATURAL HISTORY. 
88 
[east. zool. 
the cavity of the Magilus aqd some other univalves, thus explaining 
what has hitherto been an anomaly in the structure of the apical pro¬ 
cesses, called thunderbolts, so common in collections. The genera 
of this family differ in the developement of the external coats of 
the apical process, and the comparative size of the chambered cavitv 
compared with the size of it, and also in the position of the syphon 
in the chamber part, as compared with the position of the shell in the 
animal. In Belemnotheutis the apical process is broad and short, only 
covered with a few thin coats. In other genera it is elongate and 
covered with numerous layers, and the layers as they are deposited 
assume a crystalline texture, so that when the apex is broken across it 
exhibits a number of crystals diverging from the centre to the cir¬ 
cumference, crossing the lines of growth. In BelemniteUa the front of 
the sheath of the apex is slit; they are only found in the chalk. In the 
other genera the sheath of the apex or alveolus is entire or only marked 
with a groove in front. In some of them, as Gastrosiphone , the syphon 
is in the front, and in another, the Notosiphone, in the dorsal part of 
the septa of the alveolus. The alveolus is sometimes obliterated. 
From the imperfect specimens of the animals of the genus Spirilla 
which have been seen, and from the very small size of the terminal 
chamber, the tenuity of the structure, and the colour of the shell, there 
is every reason to believe that it will form a family of this oj-der ; and 
if this should be the case, it is very probable, as the shells have the same 
texture, an equally small terminal chamber, and a very contracted 
mouth, that the fossil Ammonites , Scaphites, and other allied genera, 
will also be referrible to this order, or to a new 7 one characterized by the 
developement of the shell, aud regular syphoned septa. The shells of 
the family Ammonitid^ are to be distinguished from Spirulid^e by 
the edge of the chamber being sinuated and the syphon placed near the 
dorsal surface. The Ammonites have a regularly spiral and discoidal 
shell. The Crioceras differs from them in the whorls not being close 
together, but quite separate from one another. The Toxoceras are 
simply curved like an Ibex’s horn. The Ancyloceras are like Crioperaces 
with the shell near the mouth produced into a straight line, and then 
bent back again towards the tip. The Scaphites differ from the latter 
only in the whorls of the spiral part being close together like the 
Ammonites . The Hamites , on the other hand, have the apical part of 
the shell bent as in the upper part of Ancyloceras , instead of being spiral, 
and the Baculites in the shell being quite straight. The TurrHites 
are spiral, and more or less turreted shells, the whorls close together, and 
the Helicoceras differ from them in the w 7 horls being separated and few 
in number, so that the shell resembles a spiral sheep’s horn. These 
shells vary much in their external form as they increase in size, some 
losing as they grow many of the spines and processes w'hich ornament 
them in their younger states. The Planites are peculiar for having a 
large ear-shaped lobe on each side of the mouth, which is said to dis¬ 
appear in the older specimens. 
The Nautilophora have an oblong body inclosed in the cavity of 
the last whorl of a chambered shell, which has a tubular syphon passing 
through each of the chambers; their eyes are slightly pedicelled, and 
they have eight arms, with numerous divisions, furnished with short 
tubular retractile tentacles and four gills. 
