136 GENESIS OF THE AKIETIDiE. 



Schlotheimia Speziana, Canavari. 



Mgoc. Spezianum, Canav., TJnt. Lias'v. Spezia, p. 166, pi. xviii. fig. 12. 



Schlot. Spezianum, Canav., Fauna del Lias, Mem. del. Carta Geol. d' Italia, III., 1888. 



This form is more compressed, has different pilations, and a remarkably 

 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 discoiclal, 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 pilse 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 geniculae, 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 Arietidas. 



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 pilae 

 fold-like, and without geniculae 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 Psil. 

 planorhe of Central Europe. The abdominal lobe, however, as in Psiloceras, is 

 not usually deeper than the superior laterals. 



