294 THE LIAS AMMONITES. 



calculated from several specimens are — height of the last whorl 1-0% ; width, -^^ ; width 

 of the umbilicus, ^% ; involution of the whorl, -^^ of the diameter of the shell. 



Description. — This Ammonite has a discoidal shell, moderately compressed on the 

 sides, but rotund towards the siphonal area, which is furnished with a very thick obtuse 

 keel, having on each side a deep sulcus separating it from two lateral carinse formed by the 

 angles of the sides and area ; aperture compressed, bi-sinuated at the summit, and strongly 

 grooved by the turn of the spire ; whorls slightly compressed in the middle, and more so 

 towards the upper third ; provided with from twenty-five to thirty moderately prominent 

 acute ribs, which are arched, and disappear at the angle of the area before they merge 

 into the lateral sulci ; the valleys between the ribs are very regularly concave. 



The entire surface of the outer lamina of the shell is traversed by fine longitudinal 

 lines, as if they had been cut by a graver. On the specimen before me there are sixty- 

 six such lines on the area and sides of the whorl, and at the point where they intersect 

 similar transverse lines a punctated depression is produced, which imparts a highly- 

 ornamental character to the shell of this species. 



The septa are symmetrical, divided on each side into four lobes and four saddles, formed 

 of unequal parts (PI. XXI, fig. 5). The siphonal lobe is long and wide, and divided on 

 the median line one third of its length, presenting numerous small equal-sized digitations 

 around its circumference ; the siphonal saddle, as large as the siphonal lobe, has shallow 

 ramifications on its outer side, and three small festoons on its anterior border ; the principal 

 lateral lobe is conical, and about two thirds the length of the siphonal lobe ; the superior 

 is larger than the siphonal saddle, and presents better developed simple festoons on its 

 border than those on the latter ; the inferior lateral lobe is about as large as the principal, 

 and has numerous small round digitations on its circumference ; the auxiliary saddle is 

 about one fourth the size of the siphonal, and has a similar form ; the auxiliary lobe is 

 nearly as large as the lateral. 



Affinities and Differences. — This species very much resembles Arietites stellaris. Sow. ; 

 so much so, that the one is often mistaken for the other. Both have longitudinal and 

 transverse lines on the shell, and punctations at their points of intersection ; in A. stellaris 

 the shell is more compressed and bevelled, the keel smaller and less obtuse ; the digita- 

 tions of the lobes more complicated, and the saddles longer and narrower. 



Locality and Stratigr af Ideal Position. — This Ammonite characterises a well-marked 

 zone of life in the Lower Lias, which was well shown at Bredon during the cutting of 

 the Bristol and Birmingham Railway, from whence I obtained the fine figured specimens. 

 The rock consisted of dark grey-bluish shales and marls, with irregular and inconstant 

 bands of limestone. This Ammonite is found near Stratford-on-Avon, Warwickshire ; 

 it is collected in great abundance between Lyme Regis and Charmouth, where the shells 

 are replaced and their chambers filled with crystallised carbonate of lime. These beautiful 

 fossils are called " Tortoise Ammonites " by the local collectors. The young examples 

 of this species found with A. planicosta, Sow., in the Marston marble of Somersetshire, 



