Fig. 259.—A, Turrilites catenulatus d’Orb, Gault; B, Macroscaphites Iranii d’Orb, Upper Neocomian. (From Zittel.)
Fam. 11. Stephanoceratidae.—Shell discoidal, helicoid or straight, whorls sometimes disunited, surface often with bifurcating ribs, which are tubercled, aperture often with lateral projections, sutural line incised, aptychus in two pieces, sometimes united.
In the discoidal group, Stephanoceras is strongly ribbed, tubercled at the point of bifurcation, Cosmoceras has long lateral projections of the aperture when young, Perisphinctes has a large body-chamber and numerous smooth ribs. Other genera are Acanthoceras, Peltoceras, Aspidoceras, and Hoplites. Among the loosely whorled genera, Scaphites (Fig. [260], A) has the last whorl produced and bent back again in horse-shoe form, while the early whorls are concealed; Hamites, Hamulina, and Ptychoceras have a shell shaped like a single or double hook, the sides of which may or may not be united; Crioceras (Fig. [260], B) in form of whorls resembles a Spirula, Ancyloceras a Scaphites with the first whorls disunited. Macroscaphites (Fig. [259], B) is similar, but with the first whorls united and not concealed. Turrilites (Fig. [259], A) is turreted and sinistral, while Baculites is quite straight, with a long body-chamber.
Fig. 260.—A, Scaphites aequalis Sowb., Cretaceous; B, Crioceras bifurcatum Quenst., Cretaceous. (From Zittel.)
CHAPTER XIV
CLASS GASTEROPODA—AMPHINEURA AND PROSOBRANCHIATA
Order I. Amphineura
Bilaterally symmetrical Mollusca, anus at the terminal end of the body, dorsal tegument more or less furnished with spicules.
Sub-order 1. Polyplacophora (Chitons).—Foot co-extensive with ventral surface of the body, dorsum with eight transverse plates, articulated (except in Chitonellus), a row of ctenidia on each side between the mantle and the foot. Silurian ——.
The Chitons are found in all parts of the world, ranging in size from a length of about half an inch to six inches or more in the giant Cryptochiton. Although in the main sub-littoral, they occur at very great depths; the Challenger dredged Leptochiton benthus Hadd. at 2300 fathoms. Chiton Polii exceptionally occurs at Malta—teste MacAndrew—above sea margin, but within reach of the ripple. As a rule, the Chitons live in concealment, on the under surface of stones or in deep and narrow fissures in the rocks. When the stone to which they are attached is turned over, they crawl slowly to the side which is not exposed, as if disliking the light. An undescribed species, however, which I took at Panama, crawled quite as fast as an ordinary snail. Chiton fulvus Wood, apparently is accustomed to crawl with some rapidity. MacAndrew took it in abundance on his anchor chain in Vigo Bay every time his yacht was got under weigh. He also found it crawling in sand on the shore, to which habit is no doubt due its extreme cleanness and freedom from the foreign growths which are so characteristic of many of the species. When detached a Chiton contracts the muscles of the whole body, and rolls up into a ball like a wood-louse.