456 THE LIAS AMMONITES. 



rib between the tubercles ; siphonal area narrow, keel very thin and extremely prominent, 

 distinct from the siphuncle ; aperture compressed, sagittate, acute; suture-line tortuous, 

 four lobed. 



Dimensions. — PI. LXVII, fig. 1. Transverse diameter 140 millimetres; width of 

 umbilicus 45 millimetres; height of aperture 55 millimetres; width of aperture 27 

 millimetres. 



PI. LXVII, figs .3, 4, var. dispansum. Transverse diameter 85 millimetres ; width of 

 umbilicus 25 millimetres; height of aperture 35 millimetres; width of aperture 18 milli- 

 metres. 



PI. LXVIII. Transverse diameter 210 millimetres; width of umbilicus 75 milli- 

 metres ; height of aperture 75 millimetres. 



Description. — This beautiful Ammonite is one of the most dominant forms of the Lyt. 

 Jurense-zone in Gloucestershire, from whence all the fine examples so accurately 

 delineated in Pis. LXVII and LXVIII were collected. The shell is discoidal, much 

 compressed and excessively carinated ; the volutions, six in number, are one half involute ; 

 the whorls are ornamented with from sixty to seventy transverse sigmoidal costae, all 

 inflected towards the aperture ; most of these are fasciated, and arise in pairs from well- 

 marked tubercles developed around the inner margin, whilst others are short and single, 

 and lie between the fasciated pairs ; there is in general one simple rib between each 

 fasciculus. The siphonal area is narrow (PI. LXVII, fig. 2), in which the inflected 

 costae all terminate ; from the centre arises a very narrow, extremely prominent keel (figs. 

 1, 2, 5, 6), which is rarely preserved, it stands straight out from the shell, and is situated 

 outside and entirely distinct from the siphuncle (fig. 2), where it is well delineated. Here 

 we observe that an accidental fracture of a portion of the shell has exposed some of the 

 chambers and the siphuncle is shown in situ, beneath the outer lamina of the shell. 

 The prominent calcareous carina has been entirely broken away from the area and was 

 evidently independent of, and detached from, the siphuncle, which really lies embedded 

 within the shell itself. In PL LXVII, figs. 3, 4 I have figured an example of the dispansum 

 variety, from the sands at Frocester Hill. The sides are more convex and run out into a 

 narrower area, with a thinner and sharper keel, the lateral costae are smaller and more 

 numerous, and the fasciated bundles less pronounced. The Ammonite described as 

 Am. Beanii, Simp., is undoubtedly the same as d'Orbigny's Am. variabilis, and it was 

 the discovery of this species in the Grey Marl, zone of Lyt. Jurense at the Peak, that 

 settled the point as to the age of that bed with me. This species and Harp, striatulum, 

 which I found associated together, proved that the Frocester Sands and the Grey Marls 

 at the Peak were upon the same horizon of life. 



The aperture is narrow, and elongated (PI. LXVII, figs. 2, G), and sometimes 

 lanceolate, as in PI. LXVII, fig. 4. 



The suture-line is extremely tortuous, and in its contour developes four lobes and 

 four saddles. The siphonal lobe is as wide but shorter than the principal lateral, and is 



