ARIETITES SCIPIONIANUS. 291 



in PI. XIII. It is an adult specimen with a considerable portion of the body- 

 chamber and traces of the lobe-line. This fossil is 225 millimetres in diameter, 

 the outer whorl at the aperture is 85 millimetres in height and 56 millimetres in width ; 

 and the aperture is compressed and forms an irregular hexagon ; the two outer facets 

 describe acute angles, the two lateral and the two inner re-entering angles. The whorls 

 are widest at the umbilical, and narrowest at the outer margin, where they are bevelled 

 away into the prominent keel. At the umbilical angle there are twenty -four thick, blunt 

 ribs slightly inclined backwards, which ascend on the sides and disappear near the 

 middle of the whorl, and then leave the upper portion smooth. The row of tubercles 

 in which the lateral ribs usually terminate are absent on this more aged shell, so that the 

 outer half of the whorl is bevelled away and rises into a high, prominent keel. A portion 

 of the shell remaining on the cast shows how extremely thin the shell of this species 

 was in so large an Ammonite. The keel, likewise, is as prominent on the cast as 

 at the parts covered by shell. 



The spire is formed of angular whorls, which are one half involute ; they are all 

 strongly ribbed as they appear near the umbilicus, the upper smooth-bevelled half being 

 concealed by the involution of the spire. The body-chamber is not complete, still it 

 occupies five sevenths of the outer whorl of this specimen. 



The lobe-line is very tortuous, and forms three lobes and three saddles. The siphonal 

 lobe is long, narrow, and freely digitate on each side. The siphonal saddle is wide and 

 oblique, with numerous festoons on its long inner side (PI. XIII, fig. 3) ; the principal 

 lateral lobe is as wide, but shorter than the siphonal ; and the lateral saddle is larger than 

 the siphonal, and is oblique and festooned in like manner. The auxiliary lobes are small 

 and indistinct. 



This species attains a very great size, as may be seen in the old shells contained in 

 the Semur Museum. 



Affinities and Differences. — This Ammonite somewhat resembles A. stellaris, Sow., 

 in the compression of the sides and bevelled surface of the outer margin. The row of 

 tubercles near the siphonal area, the naiTowness of that space and entire absence 

 of sulci from each side of the keel, entirely separate it from A. stellaris, and there is 

 no other form in the Lower Lias with which it has any affinities. 



Locality and Stratigraphical Position. — It was collected from the Lower Lias Shales, 

 in the Vale of Gloucester, during the deep cuttings made in the formation of the Midland 

 Railway. In Yorkshire it is found in position in the upper part of the Bucldandi Zone 

 at Marske, Redcar ; and in Robin Hood's Bay, in blocks, associated with Lima pectitioides, 

 Lucina limbata, Bentalium elatense, indicating the same horizon there. 



In France it is found in the upper part of the zone of Arietites Bucklandi with Arietites 

 rotiformis and Ar. semicostatus. Young, and is rarely discovered in the lower layers 

 of the zone. Dumortier says it is collected at Saint-Didier, Rhone; also at Curgy, 

 Borgy, Saone-et-Loire ; Salins, Clomot, Nolay, Cote-d'Or; Mont-de-Lans, Isere; and 



