856 TETRABRANCHIATE CEPHALOPODS. 



division into a number of genera, which in turn constitute a number 

 of distinct families. These modern divisions of the old genus 

 Ammonites are distinguished by such morphological characters as 

 the size of the body-chamber, the form of the sutures, the shape of 

 the aperture, and the presence or absence of an " Aptychus." The 

 mere form of the shell is a matter to which comparatively small 

 weight is attached. Hence genera such as Baculites, Turrilites, 

 and the like, in which the most obvious peculiarity is the external 

 configuration of the shell, are now distributed, on the ground of 

 their internal structure, among the different families into which the 

 genus Ammonites has been broken up. 



The three principal genera of the Arcestidce. are Arcestes, Cyclo- 

 /obus, and Lobites. In the genus Arcestes the shell is involute, the 

 last whorl being of large size, more or less completely concealing the 

 inner whorls, from which it often differs in character. The umbilicus 

 becomes constricted in course of growth, and is commonly completely 

 closed by a callous deposit. The aperture is without lateral lobes, 

 and the surface is smooth or furnished with fine transverse striae. 

 The species of Arcestes are almost exclusively Triassic, though early 

 types {Arcestes priscus, Waagen) appear in the 

 Permo-Carboniferous rocks of India, in which 

 also is found the nearly allied genus Cyclolo- 

 bus (fig. 770). The genus Lobites agrees with 

 Arcestes in the general form of the shell, the 

 long body-chamber, and the common closure 

 of the umbilicus by a callosity. The surface, 

 however, is commonly marked with radial 

 ^Lif%iS ; " folds (fig. 771), sometimes intersected by 

 longitudinal striae. The aperture may be 

 simple, but is usually prolonged into a projecting hood. The 

 species of this genus are confined to the Trias. 



Family 4. Tropitid^. — In this family the shell is discoidal, with 

 a long body-chamber, extending over more than one whorl. The 

 surface is more or less richly ornamented, with radial ribs which 

 often carry spines or tubercles. The sutures are foliaceous, the 

 lobes and saddles being laterally incised. The principal genus of 

 this family is Tropites itself, the species of which are found in the 

 Trias principally, but survive into the Lias. 



Family 5. Ceratitid^:. — In this family the form of the shell is 

 variable, and its surface is ornamented with ribs or tubercles. The 

 body-chamber is short, not extending over more than one-half or 

 three-quarters of a volution. The sutures mostly show simple non- 

 serrated "saddles," while the "lobes" are denticulated (fig. 772). 

 The aperture is simple, and an " Aptychus " is not known to have 

 been present. 



