ARIET1TIES NODOTIANUS. 301 



Ammonites nodotianus, Studer. Geologie der Schweitz, ii, p. 35, 1854. 



— — von Hauer. Cephalopoden aus dem Lias N.-O. Alpen, p. 24, 



pi. vi, figs. 1—3, 1856. 

 Arietites? Macdonnelli, Tate and Blake. Yorksh. Lias, p. 290, pi. v, fig. 8, 18T6. 



Diagnosis. — Shell discoidal, compressed, carinate ; whorls one third involute, flat, and 

 slightly convex, sides with numerous simple, straight, flat folds, 32 — 40 in the last whorl, 

 most distinct between the umbilical border and the margin, and becoming evanescent and 

 disappearing on the siphonal area, which is narrow and acute, with a thin prominent 

 keel permanent in the cast ; aperture narrow, elongated, elliptical, and compressed ; 

 on the sides a slight depression on each side of the keel ; lobe-line simple ; septa remote 

 from each other. 



Dimensions. — Diameter of figured specimen, 65 millimetres; width of umbilicus 35 

 millimetres ; depth of the last whorl 15 millimetres; height of aperture 20 millimetres; 

 transverse diameter of ditto 11 millimetres. 



Description. — I am indebted to my old friend Professor Hull, F.R.S., for the loan 

 of General Portlock's original type of Ammonites Macdonnellii, figured in his ' Geological 

 Report of Geology of Londonderry,' and which belongs to the Dublin Museum of the 

 Geological Survey of Ireland. The drawing did not do justice to the specimen. I have, 

 therefore, had this shell carefully redrawn, with front and side views of the fossil. 



I was the first to find this Ammonite in the English Lias in the zone of Aegoceras 

 Jamesoni, sub-zone armatus, near Cheltenham. The shale in which it lies is highly 

 ferruginous, and many of the specimens are much decomposed. 



The shell is much compressed, discoidal, and strongly carinated ; the whorls, seven in 

 number, and about one third involute, are flat and only slightly convex ; the sides are 

 covered with numerous straight, simple folds or flat ribs (fig. 3), which are higher than they 

 are wide, and a transverse section forms a regular ellipse. The height of the last whorl 

 is about one fourth the diameter of the shell. The ribs, from 36 — 40 in the last 

 whorl, are slightly elevated, and in shells of 60 — 70 millimetres are merely flat folds of 

 the shell (fig. 3). In smaller specimens, 25 millimetres in diameter, they are regular 

 and delicate ribs ; about this age the keel first makes its appearance. As the shell grows, 

 it becomes a thin, slender, prominent carina, which develops into a trenchant prominence 

 on the shell, and forms (figs. 3 and 4) a well-marked carina on the cast. The ribs 

 are largest between the umbilical border and the margin, where they diminish, and 

 finally disappear on the siphonal area. The aperture is oblong, ellipsoidal, and compressed 

 at the sides. 



The lobe-line is very simple, all the lobes and saddles are slightly incised, and the 

 septa placed wide apart ; the siphonal lobe is a little longer than the principal lateral, and 

 is tridigitate on each side. The principal saddle has three unequal festoons ; the siphonal 

 lateral lobe is fivefold ; the lateral saddle is smaller, but similar to the siphonal. 

 The lateral lobe is about two-thirds the size of the principal lateral, and terminates in. 



