HARPOCERAS VARIABILE. 457 



divided by the median line up to two fifths of its height, and on each side presents three 

 points and one long terminal branch. The siphonal saddle is very wide, and divided by 

 an accessory lobe at its termination into two unequal parts, the internal being the 

 longest. The principal lateral lobe is large and pyramidal, and on each side has four 

 lateral and two long terminal branches with many points. The lateral saddle is narrow, 

 is divided by an accessory lobe into two portions, and terminates in from six to eight 

 leaf-like expansions, but on a smaller scale than those found in the siphonal saddle. The 

 inner lateral lobe is about half the size of the principal lateral, and is ornamented on 

 each side with three branches, each terminating in a long slender filament. The auxiliary 

 saddle is oblique and unequally divided, the external portion being the largest ; it 

 terminates in two lateral folioles and a terminal trifid foliole. The two auxiliary lobes 

 are very unequal, the external is long, with lateral digitations ; the inner much smaller, 

 and develops only a sharp point. The radial line touches the points of the siphonal, 

 inner lateral, and auxiliary lobes, and cuts the long terminal branches of the principal 

 lateral lobe. 



This species varies much in form at different periods of life. In youth the shell is 

 thicker and more inflated, and the keel small or absent ; aftervi'ards both carina and lateral 

 costse are evolved, the marginal tubercles soon make their appearance, and the costas 

 assume the fasciated style they long maintain, until with advancing years the shell 

 becomes flatter, the whorls more expanded, and the tubercles and costse alike disappear 

 from the surface. It cannot with justice be said, as d'Orbigny's name suggests, that its 

 specific characters vary much : on the contrary, I consider that they are all well 

 marked and stationary. 



Affinities and Differences. — We discover two distinct varieties of Harpoceras variabile. 

 The first (PI. LXVII, figs. 1 and 5) may be considered as the type a, or d' Orhignian 

 form, the second, h, (PI. LXVII, fig. 3) I shall describe as the Dispansicm form. The 

 d'Orbignian form is found in the lower sands, whereas the Dispansion variety is 

 found in the upper sands. This latter shell (PI. LXVII, figs. 3 and 4) is much more 

 discoidal, the keel more acute, the marginal tubercles smafler and more irregular, the 

 costae more delicate, sigmoidal, and fasciated. I have placed together on the same 

 plate (LXVII), and side by side, the two forms into which the species ranges itself, 

 and in PL LXVIII have figured one of the finest and largest shells I have ever seen of 

 Harp, variabile, with all its shell preserved ; it came from the upper sands. I shall 

 here quote a passage from the paper of my old friend Dr. Lycett " On the Ammonites 

 of the Sands between the Upper Lias and Inferior Oolite :"^ 



" Ammonites dispansus, Lycett. This was long confounded with A. variabilis, 



and it has only been after a comparison of very numerous specimens of both 



species, and of all stages of growth, that it has been found necessary to separate them ; 



their geological position is also quite distinct, A. dispansus occurs only in the upper 



1 ' Proceedings of the CoUeswold Naturalists Field Club,' vol. iii, p. 5. 



69 



