358 THE LIAS AMMONITES. 



margin of the siplaonal area, the other at some distance from the spiral suture ; siphonal 

 area narrow, convex, carinated ; aperture narrow, elongated, and flattened laterally. 



Dimensions. — Transverse diameter 84 millimetres; Avidth of the umbilicus 45 milli- 

 metres; height of aperture 20 millimetres ; transverse width 15 millimetres. 



Description. — This Ammonite is supposed to be the biptcndatus of Schlotheim — be 

 that as it may, it certainly is the bipunctatus of Homer, who has very accurately described 

 it in his ' Versteinerungen, &c.' The shell is discoidal, compressed, and flattened on the 

 sides of the whorls, which support two rows of small blunt tubercles, the one row bound 

 the margin of the siphonal area, the second row is developed at some distance from the 

 spiral suture ; and between the two tubercles a short, straight rib extends. The whorls 

 are for nearly one-third their height involute, and the turn of the spire just covers and 

 encloses the outer row of tubercles, whilst the inner row is visible on the whorls up to 

 the earliest turns of the spire. The shell slopes very obliquely, at an angle of 80° from the 

 inner row of tubercles to the foregoing whorl, so that the umbilicus has a stair-like 

 aspect from the flat step or side of the whorl, and the inclined riser which leads to it. I 

 know of no other Ammonite that has this feature so well shown, and which has been 

 very well represented in fig. 1 . The siphonal area is narrow and angular in consequence of 

 the development of the carina. From the outer tubercles a continuation of the ribs 

 extends obliquely forward towards the aperture, with other intermediate strise having a 

 like direction ; so that good specimens with the shell preserved have the area ornate 

 with fine oblique lines stretching forward from the marginal tubercles towards the central 

 carina. The aperture is quadrate and elongated (PI. XXXVIII, fig. 2), one third less in 

 the transverse than the vertical diameter. 



The lobe-line is extremely convoluted (fig. 4). The siphonal lobe is wider and shorter 

 than the principal lateral, and ornamented on each side with three branches, of which 

 the lowest is large and pyramidal. The siphonal saddle is wider than the principal 

 lateral lobe, and formed of two very wide, much ramified, unequal parts, the largest 

 being the innermost. The principal lateral lobe is formed on each side of three short 

 single branches inferiorly, and the two lateral formed of two branches. The lateral 

 saddle is narrower and deeper than the siphonal, and festooned into folioles which are 

 unequally divided by a projecting process, the innermost being the larger of the two 

 groups. The lateral lobe has three unequal branches, the terminal being sharply pointed. 

 The auxiliary saddle is oblique, and formed of two unequal parts, the external being the 

 larger. 



I have found the Aptychus of this species in the body-chamber of a moderate-sized 

 fragment (fig. 3) ; it has a central ridge, from which oblique lines contour the Aptychus, 

 which was a thin structure with fine striations running in an opposite direction to the 

 contouring lines of the lobe. 



This Ammonite attains a considerable magnitude. I have collected many fragments, 

 which indicated a shell at least 200 millimetres in diameter. The very small amount of 



