38 FOSSIL MOLLUSCA OF THE CHALK. 



when young, the ribs are about twelve in number, nearly equal, each bearing seven 

 tubercles ; one large, close to the umbilicus ; another, very large and conical, near the 

 side of the back ; and three on the back, which are smaller and compressed laterally : in 

 the middle stage there is occasionally one small rib, bearing three or five tubercles, 

 between the larger ones ; and in the adult shell there are usually two such smaller tuber- 

 culated ribs between the principal ones, making about twenty ribs on the whorl, and the 

 tubercles are very unequal in size : back very broad : umbilicus small and deep, exposing 

 half the inner whorls : aperture transversely rhomboidal : septa with a very broad dorsal 

 saddle, and two lateral lobes, of which the superior is bifid ; the other has not been well 

 seen. 



Diameter, 2^ inches; height of last whorl, 1^ inch; width of aperture, 1^ inch. 



Tolerably common in the Chalk with silicious grains of Lyme Regis, Dorsetshire ; 

 and Chardstock, Somersetshire. 



Many of the Continental authors have united this species with A. Rhotomagensis, 

 probably without having seen the shell, which appears to be confined to one bed in the 

 West of England. When very young the two shells are nearly alike ; but ^4. hippocastanum 

 may be known by its greater breadth of whorl, and large lateral spines. As it increases 

 in size, it is distinguished by the rapid enlargement of its whorls, great breadth, and great 

 inequality of its ribs and tubercles ; while A. Rhotomage?isis is, at all its stages, known by 

 their uniformity. A. Cunningtoni has more affinity to A. hippocastanum ; but not having 

 seen the young state of that shell, I am unable to compare them thoroughly. 



24. Ammonites Goupilianus, D'Orbigny. Plate XVII, figs. 5, 6. 



Ammonites Goupilianus, TfOrhigny. Paluont. Franc;. Terr. CreU, t. xciv, figs. 1 — 3. 



A. testa discoided, compressd, carinatd, transversim undaio-costatd ; costis depressis, 

 numerosis inaqualibus bicurvatis : dorso obiuso, carinato, producto : anfractibus tribus, ad 

 umbilicum declivibus, sub-carinatis, deinde sub-sulcatis, compressis : umbilico mediocri .• 

 aperturd elongato-ovatd : septorum lobis lateralibus 6, trijidis. 



Shell discoidal, flattened on the sides, keeled ; ornamented with numerous unequal, 

 flexuous ribs, which, rising from the raised edge of the umbilicus, incline forward for two 

 fifths of the width of the whorl, then bend slightly backwards, and again bend forward 

 near the back, and are produced forward at the keel, marking the outline of the mouth of 

 the shell at its successive stages of growth ; a few of the ribs are stouter than the rest : 

 umbilicus of moderate size, exposing about one fourth of the inner whorls, and bounded 

 by nearly perpendicular walls : sides of the whorls depressed into a slight channel near the 

 umbilicus : back obtuse, with a sharp keel, and projecting in a point over the oval mouth : 

 septa with six trifid lateral lobes. In the youngest stage of growth seen, the larger ribs 



