82 AMMONITID. 
ceras, usually somewhat more deeply notched, with a dependent 
sutural lobe. Aptychus divided, calcareous, very thin, exter- 
nally granular. 
This group corresponds to the Planulati of Buch. 
Srmoceras, Zittel. 
Distr.—26 sp. Jurassic. S. Jooraensis, Waagen (xli, 66, 67). 
Shell very flat, discoidal, umbilicus wide, with numerous whorls, 
which increase in thickness very slowly (except in the geologi- 
cally oldest forms) ; external side rounded or grooved ; sculpture 
seldom absent, consisting mostly of straight, simple or forked 
ribs, which are interrupted during most of the lifetime of the 
animal; interrupted at any rate in the young state on the exter- 
nal side, and which are often ornamented with tubercles or 
strongly swollen on the last whorl; isolated constrictions directed 
forwards on all the whorls. Body-chamber long, at least three- 
quarters of a whorl, usually attaining a greater length. Lobular 
line not very complicated. Siphonal lobe largest, external saddle 
much developed and broad, laterals one-pointed, very small 
in the geologically younger forms. Aptychus (°?). 
REINECKEIA, Bayle, 1878. Shell close to Perisphinctes, but 
with more marked constrictions, and a peripheral groove. S. 
anceps, Reinecke. 
PEeELTOCERAS, Waagen, 1871. 
Distr.—13 sp. Jurassic; Europe, India. P. Arduennense, 
dOrb. (xh, 64, 65). 
Shell when young having the ornamentation of Perisphinctes ; 
when adult, with straight tuberculated ribs; aperture with per- 
sistent lateral pr ojections. 
AspIDOcERAS, Zittel, 1866. 
Syn.—Waagenia, Neumayr, 1878. 
Distr.—48 sp. Jurassic; Cretaceous. A. longispinum, Sowb. 
(xxxiv, 56, 58). 
External form very variable, sometimes flat with wide umbil- 
icus, sometimes inflated with a narrow umbilicus; external side 
rounded or with a broad external furrow, never with a carina or 
angle. Sculpture consisting of one or two rows of tubercles or 
wanting. Ribs, as a rule, present only in the young state. Mar- 
gin of aperture simple (Asp. aporum with ears ?), body-chamber 
short, embracing two-thirds of a whorl. Lobular line tolerably 
simple; siphonal, two laterals, also often (in the geologically 
younger species) an auxiliary lobe. Lobes not much cut (with 
the exception of Asp. Altenense and circumspinosum) ; bodies of 
the lobes and saddles broad. Cellulose aptychi. 
Aspidoceras reaches the highest point of its development in 
the Kimmeridgian, and dies out in the Neocomian. 
