136 GENESIS OF THE ARIETIDA. 
Schlotheimia Speziana, Canavart. 
Zigoc. Spezianum, Canav., Unt. Lias v. Spezia, p. 166, pl. xviii. fig. 12. 
Schlot. Spezianum, Canav., Fauna del Lias, Mem. del. Carta Geol. @ Italia, III., 1888. 
This form is more compressed, has different pilations, and a remarkabl 
P ? ) 
narrow channel. It is also as involute as any other species of this subseries, and 
appears in the drawing to exceed all other forms in this respect. 
THIRD, OR VERMICERAN BRANCH. 
The living chamber is in most species attenuated, cylindrical, at least a volu- 
tion in length, and sometimes over one and a half volutions. The shell in the 
majority of forms is discoidal, and the area of envelopment almost invariably 
limited to the abdomen. During the senile stages, the whorl tends to acquire 
smooth, rounded, and convergent sides, and frequently loses the keel and channels, 
thus completely reverting to the smooth cylindrical form of the young. The 
sutures in each separate series tend to increase in the depth and breadth of the 
second lateral saddles in the higher species, and there is a backward trend of the 
auxiliary lobes and saddles which is strongly marked in some forms. In Vermi- 
ceras the sutures become more decidedly arietian, the abdominal lobe is deeper 
and narrower, the lateral lobes are broader and less dendritic, and the auxiliary 
lobes and saddles are not as a rule inclined posteriorly. 
CALOCERAS. 
The shells are extremely discoidal, with numerous almost cylindrical whorls 
which often retain the nealogic form throughout life. The pile are curved, and 
they usually have an immature fold-like character, in keeping with the arrested 
development of the form in the adult whorls. They also do not have well de- 
fined genicule, and in some species they may be straight, and even tuberculated. 
The sutures usually have longer and narrower lobes, deeper saddles, and more 
complicated margins than in Psiloceras. They are, however, hardly distinguish- 
able generically from those of some species of that genus, which occur in the 
Northeastern Alps, and in some species also they approximate to those of 
Vermiceras. The range of form and characteristics is very great, as might be 
imagined, in a group which is transitional from Psiloceras to the true Arietide. 
First SUBSERIES. 
The whorls are rounded and gibbous, the keel when present not prominent ; 
the channels absent or appearing merely as smooth, inflected zones; the pile 
fold-like, and without genicule or tubercules. The sutures are very variable, 
some having more complicated margins, as in the Psiloceratites of the North- 
eastern Alps, and others approximating more nearly to the simplicity of Psi/. 
planorbe of Central Europe. The abdominal lobe, however, as in Psiloceras, is 
not usually deeper than the superior laterals, 
