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162 GENESIS OF THE ARIETIDA. 
The form in adults is discoidal, but the whorl is quadragonal. The adult 
shell is discoidal ; no involute forms have been found. The pile are prominent, 
thin, sharp, straight, and smooth;’ the genicule very abrupt, and on a level 
with the abdomen. 
The sutures have immature margins, but an arietian aspect. The siphonal 
saddles are large and pointed; the abdominal lobe may be either equal to or 
much shorter than the superior lateral lobes. The latter are remarkably large 
and long, and the inferior lateral lobes short. This gives an elevated aspect to 
this portion of the suture. The superior lateral saddles are more distinctly bifid 
in this genus than in any other, owing to the absence, as a usual thing, of the 
accompanying marginal lobes of large size. The sutural margins are generally 
smooth or simply serrated, instead of more or less foliaceous. 
The living chambers may be from one half to one volution in length. 
Aged specimens are very rare, though the species are well represented by 
individuals. Indications of the approach of senility have been seen im some 
specimens, and the geratologous metamorphoses were probably similar to those 
of Vermiceras. Such a giant, however, as Amm. Arnouldi, Dum., figured in the 
“ ftudes Pal. Bassin du Rhone,” Plate VI, which was 274 mm. in diameter, is 
not described or figured as affected by senile metamorphoses, and the huge 
Amm. geometricus, Dum., Plate XXX., which was 162, mm. in diameter, had a 
similar history. 
First SUBSERIES. 
Arnioceras miserabile, Hyarr. 
Plate Il. Fig. 4-7. Summ. Pl. XII. Fig. 2. 
Var. acutidorsale. 
Plate II. Fig. 4-6. 
Psil. acutidorsale, Hyarr, Bull. Mus. Comp. Zool., I., No. 5, p. 73. 
Amm. miserabile, Quenst., Amm. Schwab. Jura, pl. xiii. fig. 27-380. 
Ariet. nodotianus, Wrieut, Lias Amm., pl. xxxvii. fig. 4, p. 300. 
Amm. Macdonelli, Porti., Geol. Rep. Londonderry, p. 184, pl. xxix. A, fig. 12. 
Locality. — Semur. 
The lobes and saddles of one specimen in the Museum colléction from 
Semur are shallow and broad; the inferior lateral saddles, however, taper to a 
blunt point. The superior lateral saddles are divided by marginal lobes more 
deeply than the inferior laterals, which are only serrated. The auxiliary lobes 
are smooth, the superior laterals deeply serrated. 
The shell is smooth for the first four and three quarters volutions. Very 
obscure folds then begin to appear near the umbilical shoulders, and on the 
second quarter of the fifth whorl reach half-way across the side. These still 
remain, however, more prominent near the umbilicus, and are less prominent 
near the abdomen, which, with the exception of the keel, is perfectly smooth. 
1 The American species Arn. Nevadanum has tubercles, but it is not yet unquestionably settled that 
this is an arnioceran form, 
2 See Arn. Macdonelli, page 164. 
