HARPOCERAS AALENSE. 459 



diameter 39 millimetres; height of whorl 16 millimetres; width of umbilicus 14 milli- 

 metres ; width of aperture 9 millimetres. 



Description. — This Ammonite has long been a puzzle to naturalists, seeing that the 

 young shells agreed very well with Zieten's pi. xxviii, fig. 3, whilst the adult shells 

 approached Harpoceras opalinum. I therefore, by the kindness of Professor Eraas, of 

 Stuttgart, obtained some type specimens of Am. Aalensis collected from the Grey Marl of 

 the Lias beds of Wasserelfingen near Aalen, from whence Zieten's type was derived, 

 and on comparing these German specimens with a series obtained from the Ammonite 

 bed of Erocester Hill, I found them to be identical with shells of the same size, and that 

 the large shell which my late friend Dr. Lycett figured in his handbook as a new species 

 under the name Am. Moorei, was in fact the adult condition of Harpoceras Aalense, 

 Zieten (PI. LXXV, fig. 9). In order to understand the history of this species we must 

 study i7«r/). Aalense in its young and adult conditions (PI. LXXXIl, fig. 1, PI. LXXX, 

 fig. 1). In youth the radii are larger and wider apart, of unequal size, and always simple, 

 the carina is acute and distinct, and the striae are sometimes grouped in bundles 

 together near the inner edge (PI. LXXXIl, fig. 1). When the shell reaches 30 milli- 

 metres in diameter an important change takes place ; the striae become smaller and more 

 closely approximated, they cover the sides and pass over the carina in close apposition ; 

 the change from the large costse of the young shell to the fine striae of the outer whorl of 

 the larger forms is very sudden, and affords a strong contrast, when seen within the 

 umbilicus. 



In the adult, so well figured in PI. LXXX, figs. 1—3, and in PI. LXXV, figs. 9, 10, 

 the beauty and regularity of this species are well delineated. The shell is discoidal, much 

 depressed, and carinated ; the whorls are two fifths involute ; the sides covered with 

 very numerous, small, single, biflexed striae, which become hair-like and more closely 

 set together in larger shells ; so numerous are they, in fact, in the specimen I have figured, 

 that they are counted with difficulty on the body-chamber. The whorls slope towards 

 the previous volution, and are equally rounded towards the carina, which is not elevated or 

 distinct from the area, and forms only its acute summit. The suture-line (PI. LXXX, fig. 3) 

 resembles that of Harp, radians (PI. LXIV, fig. 4). The siphonal lobe is short and wide, 

 it has several lateral digitations and two divergent terminal points. The siphonal saddle is 

 wide, and divided by a small accessory lobe into two nearly equal spaces, which terminate 

 in three folioles on each side. The principal latei'al lobe is large ; with three lateral digits 

 on each side and two terminal points at its summit. The lateral saddle is small, and the 

 inner lateral lobe, about one third the size of the principal, has a few digits on its circum- 

 ference. The accessory saddle is a shallow space leading to two small auxiliary points. 



The test of Harp. Aalense is very thin, and in general well preserved. The mould 

 exhibits the form and direction of the radii in great perfection, better, in fact, than in 

 those specimens which have retained their test. 



Affinities and Differences. — Harp. Aalense, Ziet., resembles Harp, radians, Rein. ; it 



