62 FOSSIL MOLLUSCA OF THE CHALK. 



calculated. The specimen of this species figured in the 'Mineral Conchology ' has a 

 diameter of 5 inches in the lower whorl, and must have been above 18 inches loner. The 

 common-sized specimens have their largest diameter about 2^ inches, and probably 

 reached 9 or 10 inches in length. 



Common in the Grey Chalk of the South of England, and in the Chloritic Marl of the 

 Isle of Wight, and the Chalk with siliceous grains, of Dorset and Somerset. 



2. Turrilites Gravesianus, B'Orbigny. Plate XXV, fig. 7, and Plate XXVI, fig. 14. 



Turetlites Gravesianus, D'Orb. Pal. Fran^ Terr. Cret., t. 144, figs. 3 — 5. 

 — tuberculatus, Mantell. Geol. of Sussex, t. 24, fig. 6. 



T. testa turrito-conicd, sinistrorsd ,• anfractibus paucioribus, extra angulato-convexis, 

 tuberculato spinosis ; tuberculis 4 serialibus ; serie superiore, in medio anfractuum, tuber- 

 culis 10 — 12 maximis, spinosis, distantibus ; seriebus tribus inferioribus prope suturam 

 tubercidis parvis, numerosis, approximates ; angulo spirali 30°. 



Shell conical, with a spiral angle of about 30°, sinistral : whorls few, externally convex, 

 and somewhat angular in the middle, with one row of 10 or 12 very large spinose tuber- 

 cles a little above their middles, and three rows of numerous small tubercles near their 

 base ; the two lowest of these rows are so near together that the tubercles almost coalesce, 

 and are partially concealed by the suture ; the tubercles of the upper row have a rounded 

 base, those of the other three rows are elongated in a direction parallel to the suture, and 

 are of the same number in each row : upper and lower sides of the whorls ornamented with 

 radiating ribs, corresponding to the tubercles of the lowest row : umbilicus narrow : section 

 of the whorls somewhat pentagonal : the perfect mouth has not been seen. 



The lobes of the septa are much divided, and form a very complicated pattern; the 

 dorsal lobe, dorsal saddle, and nearly two thirds of the superior lateral lobe are visible on 

 the outer side of the whorl ; the latter has four principal branches, of very unequal size : 

 the dorsal lobe is short ; the dorsal saddle is very large, and unevenly divided, the upper 

 half having two principal branches, of which the upper ends in three, the lower in two 

 terminal branchlets ; the lower half of the dorsal saddle has only one undivided straight 

 branch, parallel to the siphuncle : this peculiarity in the dorsal saddle distinguishes the 

 species from T. tuberculatus ; the superior lateral lobe is very large, and is divided into 

 five large branches (Plate XXVI, fig. 14). 



Diameter of the largest whorl, If inch ; presumed length, 5 inches. 



Rare in the Grey Chalk, near Lewes, and in the Chloritic Marl of Ventnor, Isle of 

 Wight. 



The external form of this species is very similar to that of T. tuberculatus, with which 



