MODERN CLASSIFICATION. 



225 



band is more voluminous, and in like manner, as the mouth was developed onwards, the 

 receding band formed the ribs and tubercles so well shown in that Ammonite. In 

 StepJum. Braikenridgn the sides of the border develop long, inflected, lateral lappets, 

 which bend over the aperture, as seen in figs. 117 and IIS. A like form of mouth- 

 border is seen in the shells of Sfephan. Sauzei and StepJian. linguiferum, and a still 

 more marked variation of the lateral lappets is found, in Sfephan. Gervillii, Stephan. 

 Brongniartii, all derived from the Stephanoceras Humphriesiamcm zone of the Inferior 

 Oolite, and Stephan. buUatum, from the Great Oolite. 



In Cosmoceras the sides are often highly ornamented with ribs and tubercles, which 

 bespeak a corresponding development in the mouth-border of the shell. In Cosmoceras 

 Jason (fig. 119) the lateral processes are very long, equalling two thirds the diameter of 

 the last whorl. Many years ago a large number of this species were collected from the 

 Oxford Clay near Chippenham, in a very perfect state of preservation, with the body- 

 chamber and mouth-border complete. In other species of this group I have not been 

 fortunate in finding the termination of the shell. 



^r^ ^ 



Pig. 119. 

 Cosmoceras Jason, Reinecke. 



Fig. 120. 



Hoplites ma 



Fig. 121. 



illaris, Schloth. 



^'-fci^^^^ 



Fig. 122. 

 Hoplites Mar/hii, d'Orb. 



In Hoplites mammillaris (figs. 120, 121) we have another example showing how the 

 remarkable ornamentation of some of the shells of this group depends on the develop- 

 ment of the mouth-border. Here the ridges with their spines, and the valleys with 

 their smooth surface succeed each other, owing to the advance of the calcareous oral band, 

 and its temporary cessation of growth, producing the remarkable form of shell this species 

 assumes (fig. 120). A similar condition of the mouth-border has formed the ornamented 

 shell of Hoplites Martini (fig. 122), which I have collected in the Lower Greensand 

 (Neocomian) from the Atherfield section. Isle of Wight. 



In Lytoceras the mouth is circular and without lateral processes : the shell is 

 distinguished especially among its Liassic forms for the reticulate structure of the test 

 and the prominent frills that are developed at intervals, as in Lytoceras fimbriatum 

 (fig. 123). These fimbriae originated from a periodic activity in the secreting power of the 



30 



