458 THE LIAS AMMONITES. 



sands, A. variabilis does not pass higher than the lowest fossiliferous beds of the 

 sands ; both are very hmited in their vertical range, and never occupy the same horizon. 

 Both in the young and adult conditions of growth A. dispansus is always more discoidal 

 than the other; the figure of the back more especially differs in its acute keel, the tubercles 

 upon the inner margin of the volutions are much more faintly marked, and unlike tliose 

 of A. variabilis they are irregular ; they give origin to numerous fasciated sigmoid al radii ; 

 in A. variabilis the radii constitute rigid, nearly straight, and comparatively prominent 

 ribs ; the septa in A. dispansus have the lobes much more simple, less pointed, and less 

 produced ; the test is preserved very rarely and only in young specimens, it is delicate, 

 and exhibits the fine hair-hke sigmoidal radii much more distinctly than the casts. 

 Specimens and fragments are very abundant at Frocester Hill ; at Haresfield Hill they 

 are present, but are badly preserved. The largest specimen in my possession is five 

 inches and a half across, but very few exceed three inches." 



Locality and Stratigraplncal Position. — The type form I have collected from the 

 sands between the Lias and Inferior Oolite near Bridport. The specimen (PI. LXVII, 

 figs. 1 and 5) came from that locality. The Harp, dispansmn type (PI. LXVII, figs. 3, 4) 

 was collected at Frocester Hill and PL LXVIII near Nailsworth, Gloucestershire. 



Harpoceras Aalense, Zieten. PI. LXXV, figs. 8 — 10 ; PI. LXXX, figs. 1 — 3 ; 



PI. LXXXII, figs. 1—4. 



Nautilus comptus? Rdneclce. Naut. et Argon., p. 57, tab. 1, figs. 5, 6, 1818. 

 Ammonites sigmifee? Phillips. Geol. of Yorksh., pi. xiii, fig. 4, 1829. 



— Aalensis, Zieten. Versteiner. Wurtemberg, p. 37, pi. xxviii, fig. 3, 1830. 



— KUGATULUS, Simpson. Fossils of Yorksh. Lias, p. 73, 1855. 



— Aalensis, Oppel. Die Juraformation, p. 248, 1856. 



— MooEEi, Lycett. Cotteswold Hills Handbook, p. 122, pi. i, fig. 2, 



1857. 

 Harpoceeas Aalense, Tate and Blake. Yorksh. Lias, p„ 303, 1876. 



Diagnosis. — a. Adult condition. — Shell discoidal, much depressed, carinated ; whorls 

 compressed, two fifths involute, inner border untruncated, sides slightly convex, closely 

 covered with fine, simple, irregular striae bent forward and inclined toward the carina, 

 which is neither elevated nor distinct. 



b. Young condition. — Striae much larger, more unequal, distant, and distinct ; siphonal 

 area thinner and more trenchant ; aperture narrow, elliptical. 



Dimensions. — Large specimen, PI. LXXX, figs. 1, 2 {== Ammonites Moorei, Lycett) : 

 transverse diameter 85 millimetres ; height of whorl 80 millimetres ; width of umbilicus 

 35 millimetres; width of aperture 15 millimetres. 



Small specimen (PI. LXXXII, figs. 1, 3, Ammonites Aalensis, Zieten) transverse 



