460 THE LIAS AMMONITES. 



is, however, more compressed, the siphonal area more acute, without a distinct keel, 

 and its fine, hair-like striae pass over the ridge ; the radii, likewise, are more unequal 

 in size, and differ much during their evolution ; the difference observed in the character 

 of the radii in young and old shells forms another point of comparison which ought 

 not to be omitted in treating of the affinities of this beautiful Ammonite, which is still 

 more closely related to Harpoceras opalinum. A critical comparison of Harp. Aalense 

 with Harp, opalinum will, however, be more in place in the article devoted to the 

 description of that most interesting and beautiful form (p. 463). 



The young forms of Harp. Aalense present a wide range of variability in 

 the ornamentation of the shell, and it is only after the acquisition of a number of 

 examples of genuine types, and a critical comparison of these with one another, 

 that we realise the fact that we have first to learn the morphology of the species 

 before we can study the varieties. In PI. LXXXII, figs. 1 — 4, I have given accurate 

 delineations of two examples of Harp. Aalense from the Lias beds of Wasserelfingen 

 near Aalen, a deposit and locality from whence Zieten's type specimens were collected. 

 These shells lead up, by a series of changes, from the young form (PI. LXXXII, fig. 

 3) through a largely costated variety to a smaller costated form and up to a still 

 finer ribbed variety which makes a transition link to the adult form presented 

 by some of the smaller forms of Harp. Moorei (PI. LXXX, fig. 1). Now, all these 

 varieties I have in years past collected from the upper sands of Frocester Hill, and a 

 few from a like bed at Nailsworth, Gloucestershire, and from Burton, Bradstock, and 

 Chideock Hill, Dorsetshire. The English and the German varieties match each other so 

 closely that we are bound to admit the forms mentioned are the necessary changes the 

 organism has passed through in the evolution of the species. 



Reinecke appears to have figured the young form with fine costa? under the name of 

 Nautilus comptus, and Lycett a variety under that of Am. Moorei, whilst Zieten has figured 

 the most typical shell (PI. LXXV, fig. 8) as Am. Aalensis. In truth, all these forms are 

 readily united by the intermediate links which one finds in a tray full of examples 

 obtained from the Upper Lias Sands. 



The variety " Harp, comptum " of Reinecke is in the young state distinguished 

 by having fine, closely arranged, sigmoidal radii, which are fasciated at irregular intervals 

 by folds of growth ; subsequently the fasciated character disappears in older forms. 



Locality and Stratigraphical Position. — Harpoceras Aalense is limited to the Opalinum 

 zone of Frocester Hill and Nailsworth, and is found in the same horizon at Burton 

 Bradstock and Chideock Hill, Dorsetshire. This Ammonite is a leading fossil in the 

 upper part of the Lyt. Jurense-marl at Wasserelfingen near Aalen, and this same stratum 

 has a wide distribution through Germany, as at Gundershoffen, and Uhrviller, Bas-Rhin ; 

 in Prance at Saint-Quintin, Isere, and the environs of Saint-Rambert, Ain ; Saint-Romain, 

 Rhone ; and Verpilliere, Isere, where magnificent specimens are obtained from the iron- 

 stone bands. 



