12 FOSSIL MOLLUSCA OF THE CHALK. 



in older specimens they end at the side of the back, and there is a slightly elevated ridge 

 along the middle of the back, with a row of tubercles on each side ; these dorsal tubercles 

 are elongated transversely in the direction of the rib's : umbilicus shallow, leaving nearly 

 half of the inner whorls visible : aperture nearly elliptical. 



Diameter of the figured specimen from the Grey Chalk, 1-^ inch 5 height of the last 

 whorl, T 6 7 inch; width of aperture, T 4 ¥ inch. 



Rare in the Grey Chalk of Ventnor, Isle of Wight. It is also found in the Upper 

 Green Sand, near Dorking, where it attains a diameter of 4 inches. In France it has 

 been found in the lower part of the "Craie Chloritee." 



Not having seen the young stage of any specimens of A. Renauxianus found in the 

 Upper Green Sand, it is with some hesitation that I unite to that species these small 

 shells found in the Lower Chalk. On a side view, they closely resemble A. varians, from 

 which they differ in the form of the back, which never rises into a decided keel as in that 

 species. The dorsal tubercles are also different, as they slope with the rib ; while in A. 

 varians they are compressed transversely to the rib, and parallel to the keel. 



29. Ammonites octo-sulcatus, Sharpe. Plate XIX, fig. 3. 



A. testa discoided : anfractibus complanatis ; dorso rotundatis, subtrilissime costatiS' 

 sulcis 8 transversis, rectiusculis, impressis ; costisnumerosis, brevibus, dorsum awplectentibus : 

 umbilico mediocri .- aperturd semi-ovatd. 



Shell discoidal ; with few whorls, flattened on the sides, and rounded on the back, 

 crossed by eight deep, very slightly flexuous furrows, between which are many short, 

 delicate ribs bending forward, and well defined on the back, fading away in the middle of 

 the sides of the whorl : umbilicus rather small, leaving a fourth of the inner whorls visible : 

 aperture semi-oval. 



Diameter, 1 inch ; width of aperture, § inch ; height of last whorl, T 4 ff inch. 



Found in the Grey Chalk of Ventnor, in the Isle of Wight, and in the Chalk with 

 silicious grains, of Chardstock. 



This species is nearly allied to A. planulatus and A. Mayorianus, of the Lower Chalk, 

 and to A. Guettardi of the Neocomian of the South of France. It differs from the first 

 two in having the constrictions more numerous, and the inner whorls more concealed ; 

 and from the last, in having the furrows straighter, and projecting more forward, a rather 

 larger umbilicus, and flatter sides to the whorl. Unfortunately the septa have not been 

 seen. A. Bolus, D'Orbigny, has a similar form and ornaments, but the furrows incline 

 backwards on crossing the back ; while in all the other species just mentioned, they project 

 forwards at the middle of the back. 



