THE CEPHALOPODA. 
189 
Aegoceras capricornum, Schloth (fig. 56). 
— catenatum, Sow. 
— Charmassii, d'Orbig. 
Aegoceras Henleyi, Sow. 
— planicosta, Soio. 
— planorbis, Sow. 
V. Planulati. — The siphonal area is always perfectly round, and is joined to 
the sides without a border. The whorls lie almost on the same plane, whereby all 
Fig. 57. — Stephanoceras annulatum Sow. • Fig. 58. — Stephanoceras commune, Sow. 
the species receive a more or less strikingly discoidal form. The numerous close-set 
ribs divide near the half or two thirds of their height into two, three, or many small folds, 
without any tubercle at the point of bifurcation. The lower portion of the rib is 
thicker and directed backwards, as in the modern genus Stephanoceras, which is the type 
of this group. The disposition of the lobes is very peculiar and complicated in the 
Planatuli. The upper lateral is large and very deeply sunk, and occupies the middle 
of the side ; the lower lateral is smaller, and there proceeds from it two or three 
auxiliary lobes with branches directed obliquely backwards or sometimes even horizontal, 
and these form together a compound lobe larger than the lower lateral, under which it 
retires, the whole constituting a most intricate arrangement which is very difficult to trace 
out. 
The Plantjlati are abundant in the Middle Oolites and in the Upper White 
Jurassic Limestone of Germany. They are also found in some of the beds of the Upper 
Lias. They are entirely absent from the Chalk. The following forms are characteristic 
of this group. 
Perisphinctes biplex. Sow. 
— giganteus, Sow. 
— plicatilis, Sow. 
— polygyratus, Schloth. 
Perisphinctes polyplocus, Rein. 
Stephanoceras commune. Sow. (fig. 58). 
— annulatum, Sow. (fig. 57). 
— crassum, Young and Bird. 
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