GENUS— ARIETITES. 267 



shell, as in OrfJioceras, GompJioceras ; and in the position, structure, and mode of calcifica- 

 tion of the siphuncle, as in Actinoceras, Huronia, Endoceras. In the Devonian and 

 Carboniferous rocks the Ammonoida appear as Goniatites, with the edges of their septa 

 forming angular, zigzag, transverse lines, marking the lobes and saddles ; in the Muschel- 

 kalk as Ceratites, having undulating septa vt^ith denticulate borders. In the Triassic, 

 Jurassic, and Cretaceous strata the lobes and saddles become very complicated, and the 

 shells assume a vast variety of forms, being straight, bent, curved, turbinate, or discoidal, 

 as seen in the various groups and genera of the Ammonitida . The morphology of the 

 Cephalopods has been admirably described by Kolliker, and illustrated with beautiful 

 figures ; and later important observations have been made by Grenacher and Lankester 

 " On the Embryology and Development of the Cephalopoda." 



i^a?H27j^.— AEGOCERATIDiE,! Neumayr, 1875. 



This family includes a considerable number of Ammonite groups, which have widely 

 difi'erent 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, and the suture-line is in 

 general complicated ; the siphonal lobe is large, and the principal lateral and secondary 

 laterals attain considerable dimensions. 



The siphonal 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 occulted by the last. 



In the presence of so much diversity in form and structure this family is subdivided 

 into three sections : 



I. Aegoceratites. — Arietites, Aegoceras. 



II. Harpoceratites. — Harpoceras, Oppelia, Haploceras. 



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

 ceras, Perisphinctes, Olcostephanus, ScapJiites, Hoplites, Acanthoceras, 

 StoliczJcaia, Crioceras, Heteroceras, Peltoceras, Aspidoceras. 



Section I. — Aegoceratites. 

 Ge7ius I. — Arietites, Waagen, 1869. 



Family — Aeietes, von Buck. Ueber die Ammouiten, p. 9, pi. iii, fig. 1, 1832. 



— d'Orbigny. Paleont. fran^., Ter. Cret., torn, i, p. 405, 1841. 



— Pictet. Traite de Paleontologie, 2 ed., torn, ii, p. 671, 1854. 

 Aeieten, Quenstedt. Handbuch, 2nd ed., p. 422, 1867. 



1 See pages 238 and 246 of this Monograph. 



