240 DIDYMITES, LOBITES. 



Genus DIDYMITES, V. Mojs. 



External form and length of body-chamber same as in Arcentes • 

 shell with sharp lines of growth and plicate wrinkles throughout 

 the whole length of the body-chamber to the aperture ; on the 

 inner convex surface of the shell there is a median furrow ; tlie 

 last whorl is constricted near the aperture. 



The sutural lines of the septa are formed of few-toothed 

 saddle pairs, which often alternate with single saddles. These 

 saddle pairs, as is shoAvn by projection of the spiral, corresi)ond 

 each to two saddles in the other genera of Ammonites. 



Didymites contains but a few triassic forms. 



DiDVMTTEs ANGUSTiLOBATUS, Hauer. T. 109, figs. (531. (ioK. 

 Genus LOBITES, Mojs. 



In external form and length of the body-chamber agreeing 

 with Arcesten and Didymitea. Shell usually with transverse 

 folds, which are frequently crossed by fine longitudinal strise. 

 The body whorl frequently assumes a form very different from 

 the inner ones, and not unfrequently closes the umbilicus with a 

 callus. Towards the aperture, however, and always in those 

 foi-ms with a closed umbilicus, there is a constriction which ex- 

 tends forwards in the form of small, projecting, lateral lobes. 

 The sutural lines of the septa consist of entire margined, high 

 saddles, somewhat contracted at their bases, which vary in 

 height in such a way that the second and fourth are perceptibly 

 lower than would be expected from their position. A high 

 siphonal process. 



In many forms there appears, regularly at the end of the bod}' 

 whorl and the one next to it, a portion constricted off" the 

 '• hood ; '' in other forms the aperture is simple, and only prolonged 

 anteriorly into lobe-like processes at the conv<^x portions, and 

 but little or not at all constricted. 



In Lobites the derivation from the goniatitic ancesti-y is much 

 more striking than in any other mesozoic genus, inasmuch as 

 the form of the lobes is still completely goniatitic. The ammo- 

 nitic stage is indicated in the structure of the lobes only by the 

 high siphonal process dividing the external lobe. 



