IS i GENESIS OF THE ARIETID^. 



straight, prominent dorsally, but obsolete on the edge of the abdomen. The 

 channel ridges have also disappeared, and the channels are only indicated on 

 either side of the keel. The keel, however, is persistent. 



The abdominal lobe on the latter part of the ninth volution is somewhat 

 more than one half longer than the superior laterals, and the inferior laterals 

 one half longer than the superior lateral lobes. This is a senile tendency to 

 return to larval proportions, since the proportional adult difference in length of 

 the abdominal lobe is at least three fifths. 



A specimen in the Geometricus bed, from Nurtingen, labelled nodosaries, in 

 the Museum of Stuttgardt, exhibits the characteristics of this species. Quen- 

 stedt's original of nodosaries shows that the identification of such specimens with 

 his nodosaries is not correct. It is very often regarded as the young of Cor. tri- 

 gonatum, but is far too much compressed, and whorls too small, though otherwise 

 quite similar. 



Wright's figure of an old specimen of this species under the name of Arid. 

 Crossi leaves little room for doubt that it occurs in England with the same 

 peculiar form as in Germany. Wright does not mention that there are any 

 varieties. 



Second Subseries. 



Coroniceras Sauzeanum, Hyatt. 



Plate VI. Fig. 4-14. Plate VIII. Fig. 1-3. Summ. PI. XII. Fig. 10. 



Amm. Sauzeanus, D'Orb., Terr. Jurass. Ceph., p. 304, pi. xcv. fig. 4, 5. 



Amni. spinaries, Quknst., Der Jura, pi. vii. fig. 4; Amm. Schwab. Jura, pi. ii. fig. 8-14 (fig. lo-17'. ; ). 



Localities. — Whitby, Leicestershire, Semur, Salius, Gmiiud. 



D'Orbigny's original specimen, (Plate VI. Fig. 12, 13,) with which this iden- 

 tification was made, is smooth probably throughout the first four whorls. The 

 centre was obscured, and the exact number of whorls was estimated. The abdo- 

 men is flat, with a very obscure siphonal ridge on the fifth whorl. The pila? are 

 terminated by a tubercle, which is elevated so that it stands on a level with or 

 above the abdominal continuation of the pilre. These nearly meet, and in some 

 specimens actually do cross the siphonal ridge, giving the shell a microceran 

 aspect. 



The further development of these peculiarities would lead to a form in which 

 the keel would become more distinct, but would be guarded by very shallow 

 channels, and in which also the piloe, gracefully curving as in this specimen, 

 would terminate in a tubercle standing out prominently on the edge of a flat- 

 tened abdomen. Such a form, of which the young is shown in Fig. 10, 11, 

 occurs in the same locality, and it is evidently an older individual of this 

 species. 



The sutures of this specimen are visible on the third quarter of the sixth 

 whorl. The abdominal lobe is shallower and broader than in variety Qaudryi, 

 and the inferior lateral saddles also broader proportionately. The sutures are 

 more like those of the voung of variety A, Cor. bisulcatwn. The edges of the 



