288 THE LIAS AMMONITES. 



on the sides into large obtuse festoons. The infero-lateral lobe is small and provided out- 

 wardly with three, and inwardly with two, simple digitations, independently of the median 

 point. The auxiliary saddle is very short and thrice festooned ; the line from the central 

 radius, in parting from the point of the dorsal lobe, touches the extremity of the principal 

 lateral lobe, but is very distant from the infero-lateral and auxiliaries. 



Affinities and Differences — This Ammonite retains its youthful form throughout life, 

 and more nearly resembles Arietites bisulcatus than any other of the congeneric forms in 

 the structure of its siphonal area, the general character of the shell, and the deep sulci 

 on each side of the keel. It is distinguished from A. bisulcatus, however, specifically by 

 the narrowness of the whorls, the compression of the sides, the greater number of 

 ribs destitute of tubercles, and by the marked difference in the figure of the lobe-line, 

 which is seen to be disposed in a different manner when the two species are carefully 

 compared. It resembles Arietites Turneri, Sow., and I have seen A. Bonnardii marked 

 A. Turneri in some continental museums ; even Dr. Oppel appears to have thought 

 them identical. It differs from A. Turneri, however, in having the whorls not nearly so 

 high, nor the shell so thick as in that species ; whilst the difference in the structure of 

 the lobe-line makes a marked specific distinction between these two allied forms, which 

 belong to the horizon containing the beds with Pentacrinus tuberculatus, Miller. 



Locality and Stratigraphical Position. — I have collected this species at Lyme Regis, 

 out of the hard limestone beds with Aegoceras Birchii ; in a slab before me the two 

 species are embedded together. It has been found in the same horizon in the Lower 

 Lias of Bredon, Lansdown, near Cheltenham, and in the Vale of Gloucester. 



I must refer the reader to p. 48 for an account of the Turneri-heds at Lyme Regis, 

 and a list of the fauna with which Arietites Bonnardii was associated. Aegoceras Birchii 

 may be now added to the list, as in a recent traverse I discovered the two Ammonites 

 embedded together in the same slab. 



Foreign Localities. — The type was found in the Lower Lias with Grgphaa arcuata 

 in the environs of Belley (Ain). The type specimens are in the Ecole des Mines, Paris. 

 It has likewise been found by Dr. Oppel, near Stuttgart (Wiirttemberg). 



Arietites subnodosus, 1 Young 8f Bird. PI. VI, figs. 2, 3. 



Ammonites subnodosus, Young Sf Bird. Yorkshire Coast, p. 258, 1828. 



— Simpson. MoDogr. of the Ammonites, p. 48, 1843. 



— Simpson. Fossils of Yorkshire Lias, p. 90, 1855. 



Arietites obesulus, Tate fy Blake. Yorkshire Lias, p. 284, pi. vi, fig. 2, 18/6. 



Diagnosis.— Shell discoidal; sides flat, compressed ; siphonal area broad, flat; keel 

 1 Termed nodulosus on the explanation of PI. VI, figs. 2 and 3. 



