406 THE LIAS AMMONITES. 



Family. —LYTOCERATIDJE, Neumayr, 1875. 



This family includes several genera which differ very much in external form, 

 but closely resemble each other in internal structure. The shells are elongated cones 

 rolled into cornute whorls, more or less involute, in the spire. They have a short 

 body-chamber about two thirds the length of last turn ; the mouth-aperture is circular 

 and slightly produced on the columellar side. So far they have characters in common, 

 but in the form of the shell, and in the morphological changes at different periods of life, 

 the divergence from a common type is extreme. This becomes evident when we 

 compare Lytoceras and Phylloceras with each other and these again with Hamites, 

 Turrilites, and Baculites, all of which are grouped together in the natural family 

 Lytoceratidae. 



Genus IV. — Lytoceras, Suess, 1865. 



Shell discoidal, more or less flattened ; umbilicus wide and open, exposing all the inner 

 turns of the spire ; the whorls round and loosely embracing each other. Body-chamber 

 two thirds the length of the last whorl ; mouth-border simple in the lateral and ventral 

 sides, with a lappet-like production resting on the preceding whorl at the columellar 

 side. 



The shell is highly ornate by the presence of transverse parallel 

 lines of growth which encircle the whorl, crossed at right angles by 

 longitudinal ridges, these together form a remarkable reticulate 

 structure in Lytoceras cornucopia^ Young (PI. LXXIII) . The shell 

 of several species is ornamented with wing-like elevations forming 

 prominent fringed ribs as in Lyt. fimbriatum, (Pis. LXXI and 

 LXXII), or deep intermittent depressions formed from previous 

 contractions of the aperture as in Lyt. hircinum, (PI. LXXV, fig. 

 4), or rounded ribs as Ijyt. torulosum (PI. LXXVI, fig. 4). 

 jimhriatum, Sow. The suturc-linc is very complicated and very well shown in Lyt. 



cornucopia (PI. LXXIII, fig. 3). The lobes are few in number 

 but much branched ; the lateral lobes and saddles are much divided into small, uniform, 

 symmetrical digitations; there are only two lateral, one very large principal, and a 

 small lateral, with a large columellar lobe covered by the preceding whorl, see (PI. 

 LXXIII, fig. 3). The siphonal lobe is small and narrow, and almost concealed by 

 the wide-spreading branches of the principal lateral. 



The genus Lytoceras is found first in the Trias, where it is represented by Lyt. Morloti, 

 Hauer ; Lyt. sphcerophyllum, Hauer ; Lyt. patensy Mojs. ; and Lyt. engyrum, Mojs. 



