162 GENESIS OF THE ARIETID.E. 



The form in adults is discoidal, but the whorl is quadragcraal. The adult 

 shell is discoidal ; no involute forms have been found. The pilse are prominent, 

 thin, sharp, straight, and smooth; 1 the geniculae 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 in some 

 specimens, and the geratologous metamorphoses were probably similar to those 

 of Vermiceras. Such a giant, however, as Amm. Arnouldi, Dura., figured in the 

 " Etudes 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, Dura., Plate XXX., which was 162 mm. in diameter, had a 

 similar history. 2 



First Subseries. 

 Arnioceras miserabile, Hyatt. 



Plate II. Fig. 4-7. Suinm. PI. XII. Fig. 3. 



Yak. acutidorsale. 



Plate II. Fig. 4-6. 



Psil. acutidorsnle, Hyatt, Bull. Mus. Comp. Zool., I., No. 5, p. 73. 



Amm. miserabile, Quexst., Amm. Schwab. Jura, pi. xiii. fig. 27-30. 



Ariet. nodotiamis, Wright, Lias Amm., pi. xxxvii. fig. 4, p. 300. 



Amm. Macdonelli, Portl., Geol. Rep. Londonderry, p. 131, pi. xxix. a, fig. 12. 



Locality. — Semur. 



The lobes and saddles of one specimen in the Museum collection 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 Am. Nevadanum has tubercles, but it is not yet unquestionably settled that 

 this is an arnioceran form. 



- Sne 1 rn. Macdonelli, page 1''.) 



