246 THE LIAS AMMONITES. 



IV. 'Family. — AEGOCERATiDiB, Neumayr. 



This family includes a considerable miraber of Ammonite groups which have widely 

 different forms and affinities, so that it is almost impossible to give a general description 

 of the family which shall be correct and comprehensive at the same time. 



The lobes present a great variety of ramified figures ; the suture-line is in general 

 complicated ; the siphonal lobe large, and the principal lateral and secondary laterals 

 attain considerable dimensions. 



The siphonal or ventral area is in general round and well developed ; in some there 

 is a keel with lateral channels more or less deeply grooved ; in others the shell is flattened 

 and discoidal, with a wide open umbilicus ; or it is round and inflated, with highly involute 

 whorls and a small umbilicus, sometimes nearly occluded by the last. In the presence 

 of so much diversity in form and structure this family is subdivided into three sections : 



I. Aegoceratites. — Aegoceras and Arietites. 



II. Harpoceratites. — Harpoceras, Oppelia, Haploceras. 



III. Stephanoceratites. — Stephanoceras, Cosmoceras, Ancyloceras, Baculina, Simo- 

 ceras, Perisphinctes, OlcostepJianm, ScapJiites, ■ Hoplites, Acanthoceras, Stoliczlcaia, 

 Crioceras, Heteroceras, Peltoceras, Aspidoceras. 



Genus Aegoceras, Waag. — The form of the shell in this genus is very variable ; it 

 is flat and discoidal, with a wide umbilicus, or round and involute with a small umbili- 

 cus ; in some shells the ventral side is convex and enlarged, and the sides highly orna- 

 mented with ribs, tubercles, or elongated spines ; in other species the sides have simple 

 folds or undulations, as Aeg. Jamesoni, Sow. (PL XI, figs. 4 — 6) and some are smooth, 



Fig. 149. — Aegoceras capri- Fig. 150. — Aegoceras Davai, 



eornum, Schlotli. Sow. 



as Aeg. planar bis. Sow. (PI. XIV, figs. 1 — 4). The ventral side is in general round, 

 as Aeg, capricornum, Schlot. (fig. 149), and Aeg. Baveei, Sow. (fig. 150); sometimes it 

 is grooved by a median channel, which interrupts the passage of the ribs from the right 

 to the left side of the shell, as is especially the case in Aeg. angulatum, Schlot. (PI. XIV, 



