FOSSIL MOLLUSCA OF THE CHALK. 



4. TuRRiLiTES ScHEUCHZERiANUS, Bosc. Plate XXVI, figs. 1 — 3. 



TuRBiLiTES ScHELCHZKRiAKUS, Bosc. Butfou du DeterviUe, vol. v, p. 190. 



— UNDULATUS, Sow. Min. Con., t. 75, figs. 1 — 3. 



— — Mantell. Fossils of the South Downs, t. 23, figs. 14 and 1 6 ; t. 24, 



tig. 8. 



— — Geinitz. Kreide, t. 13, figs. 1—3. 



— — UOrb. Paleont. Fran9. Terr. Cr6t., t. 146, figs. 3, 4. 



— Desnoyeusi, D'Orb. Paleont. Frau^. Terr. Cret., t. 146, figs. 1, 2. 



T. testa turritd, sinistrorsd : anfractihm nmnerosis, extra rotundatis, costatis : costis 

 numerosis, sub-anffulatls ; junioribus inierruptis ; adultis continuis : angulo spirali 

 15°— 18°. 



Shell turreted, with a spiral angle of 15° to 18°, sinistral: whorls numerous, rounded 

 externally, crossed by 20 to 24 sharp, elevated ribs ; on the upper whorls the ribs are 

 nearly straight, and broken into two unequal parts by a depression a little below their 

 middle ; towards the middle of the shell the depression gradually disappears, and on the 

 lower whorls the ribs continue uninterruptedly across the whorl, and become more and 

 more flexuous : upper and under sides of the whorls smooth : umbilicus very small : mouth 

 produced considerably do\Miwards, partially arched over, and expanded towards the 

 umbilicus : the outline of the septa has not been seen. 



Diameter of last whorl, If inch; estimated length, 8 inches. 



Common in the Grey Chalk, wherever that bed occurs, in the South of England. 



This species is so well distinguished from all the other Turrilites by its elongated form 

 and comparatively simple ribs, that it had remained free from all confusion, until M. 

 D'Orbigny unfortunately proposed the specific name of T. Desnoyersi for its youno- form, 

 and gave an appearance of probability to his suggestion by representing both the youno- 

 and the old forms as full grown shells on his plate 140, without any variation in the ribs 

 during the growth of either.* Such errors are the natural consequence of what is called 

 restoring the entire form of a shell from a fragment, instead of representing what is actually 

 seen. Dr. Mantell had already given an admirable figure of a nearly perfect specimen, 

 exhibiting the change from the broken ribs of the upper whorls to the unbroken ribs of 

 the older whorls, which ought to have preserved M. D'Orbigny from this unnecessary 

 addition of a synonym. 



Figure 2 represents a young shell, with ribs still divided, which is already beginning 



* Figures 1 and 3, plate 146, are both stated to be "restaure sur un echautillon de ma collection." 

 D'Orb., 'Terr. Cret.,' vol. i, pp. G02 and 604. 



