192 



THE LIAS AMMONITES. 



X. Dentati. — In the Argonauts the tubercles which border the narrow margin of 

 their shell are probably formed by the rising of the mantle in the shape of folds, due to the 

 sucking discs of the dorsal or back-folding arms of the female (fig. 68). So probably in the 



Fig. 68. Fig. 69. Fig. 70. 



Argonauta argo, Linn, Hoplites mammillaris, Schloth. 



Fig. 71. 

 Cosmoceras Jason, Reinecke. 



same manner might the decoration of the shell of the Dentati have been formed. The 

 tubercles grow on both sides of a flat, narrow siphonal area like a double projecting 

 wreath. They do not always stand in the direction of the radii, whereby they are especially 

 distinguished from other prominences and tubercles showing the back folding, which are 

 only strongly elevated folds or arms stretched over the shell. The lateral surfaces are 

 moderately parallel and very large, because the whorls increase, in general, very rapidly 

 in height ; at other times they are, in general, without any wider elevation or important 

 processes and points. Erom the lower part many folds ascend, which, upon half of the 

 sides, are divided, and then, sometimes, a pearl-like series of small tubercles rise up. 

 They are very elegant forms, which are pecuhar to the Upper Oolitic formations. The 

 siphoual lobe is much less deep than the upper lateral one. This less depth distinguishes 

 the Dentati from the Armati. The following species are typical forms of the group : 



Cosmoceras dentatus, Sow. I Cosmoceras Duncani, Sow. (fig. 72). 



— Jason, iJemecAe (fig. 71). | — Calloviense, Soir. 



XI. Ornati. — Spines or tubercles bound the narrow siphonal area, as in the Dentati. 

 Another series of tubercles are developed upon the middle of the sides. In the flat 

 interspace between these two rows of tubercles the upper lateral lobe is sunk, as in the 

 Armati. This plain surface is not, as in the latter, the side itself, but only a truncation 

 of the border between the margin of the area and sides. The lower lateral lobe is also 

 separated by a series of tubercles from the suture, and by a surface which converges 

 towards the columellar. The mouth-opening thereby acquires an almost regular hexagonal 

 form. These beautiful Ammonites are in general small, and peculiar to the Oxford Clay 

 and Upper Oolites. 



Cosmoceras pollux, Reinecke \ Cosmoceras castor, Reinecke. 



