AMMONOIDEA. 859 



and the saddles terminate in bladder-shaped rounded ends. An 

 " Aptychus " is not known to have been present. 



The type-genus of this family is Phylloceras, comprising involute 

 " Ammonites," with a widely expanded body-chamber, a com- 

 pressed form, and either no umbilicus or a small one (fig. 774). 

 The surface usually exhibits fine transverse striae, which are directed 

 forwards ; and the external margin is never keeled or tuberculated. 

 The aperture is simple, with short ventral lobes. The most char- 

 acteristic feature, however, is the presence of numerous saddles, 

 which gradually diminish in size from without inwards, and which 

 end in rounded or bladder-shaped extremities (fig. 776). The 

 species of Phylloceras range from the Lias to the Cretaceous 

 rocks, but the genus is preceded in the Trias by allied types 

 (Megaphyllites) . 



Family 9. Lytoceratid^e. — Shell extremely variable in shape ; 

 the body-chamber in the coiled types occupying from two-thirds to 

 three-quarters of the last volution. The surface is adorned with 

 simple, wavy, or nodose ribs, which may be straight or bent. The 



s ***** t* 



Fig. 777. — Suture-line of Lytoceras Liebigi, Jurassic, s, Siphonal lobe; es, External saddle. 



suture-line exhibits few (generally six), deeply incised lobes and 

 saddles ; the superior, and commonly also the inferior, lateral lobe 

 being divided into two symmetrical halves, while the saddles may 

 also be more or less clearly bilateral (fig. 777). An "Aptychus" 

 is only known in the single genus Baculites (Zittel). 



The type-genus of this family is Lytoceras itself, which, like all 

 the " Ammonites," possesses a discoidal shell. As shown, however, 

 by Neumayr, the characteristic suture-line of Lytoceras is possessed 

 by a number of forms, such as Turrilites, Baculites, JLamites, &c, in 

 which the shell may be coiled into a turreted spiral, or may be 

 straight, hook-shaped, &c. The shell in Lytoceras (fig. 778) is dis- 

 coidal and widely umbilicated, the whorls contiguous but hardly 

 or slightly overlapping. The surface is usually adorned with radial 

 lines of growth, or periodic contractions of the shell, with corre- 

 sponding prominent fringed ribs ("fimbriae") at intervals. The 

 aperture is simple, with an internal lappet resting on the preced- 

 ing whorl. The suture-line (fig. 777) is complex, the lateral lobes 



