Page:The New International Encyclopædia 1st ed. v. 04.djvu/506

* CEPHALOPODA. 432 CEPHALOPODA. CTca.sed, as evidenced by the somewhat larger size of the hypoiioniic sinus. The degree of evirvatiire of the shell is impor- tant. The earliest relatives of Xautilus were straight shells that had all the morphie char- acters of Xavitilus itself: soon curved and coiled forms appeared, and the coiled forms were in all cases evolved from straight forms through inter- mediate curved forms. Accompanying the changes in curvature were also changes in the form and position of the siphuncle. ( See Ok- 'riioiKK.s : ('vHToiKHAs; XAUTU.rs.) The coiled forms, such as Xautilus, etc., appeared in con- sider:) l)lc numbers at a very early age in the Lower Ordovician period, and certain nautiliconic genera continued, often in abundance, in the late Paleozoic and Jlesozoic periods, to the present time, Xautilus itself maintaining its early form without change. The straight coues (Orthoceras and its allies) began at an even ear- lier age in the Upper ('ainl)rian. They reached a great expansion ihiring the Ordovician and Silurian periods, when they gave rise to many peculiar genera, such a.s C'onoceras, Piloceras, As- coceras, Sphyradoceras, Endoceras, and Lituites; they declined tlirough the Devonian and Car- honiferous periods, and became extinct in the Triassic. Various stocks of straight shells, in assum- ing the habits of botli crawlers and swimmers, assumed curved forms and gave rise to such genera as Cyrtoi'eras, Phragmoceras, Gomphoce- ras, etc., at various jieriods in the histoiy of each stock. The sliells of some of these curved forms, through adaptation to a crawling life on the ocean bottom, became more closely coiled, their organization changed, and a new order (the Ammonoidea) arose, which outnumbered its an- cestors, the Xautiloidea, enjoyed a pyrotechnic expansicm during the early Mesozoic ])eriod, soon began to decline, and eventually, at tlu' end of the Cretaceous, became suddenly totally extinct. The ammonoidean sliclls ditTcr from the nau- tiloidean in that they always have a calcareous protoconch of sac-like form attached to the apex of the cone. On this protoconch is formed the early ammonoid shell with the septa of simple curves like those of the nautiloids. After a few chambers have been formed the septa become crimped or wriidcled, and the sutures wavy or with i)rominent cuies. In the earliest Devonian mcmlicrs of the group, the goniatitoids, these simply curved or undulating .sutures are the rule in adult shells. Later members of the lles- ozoic Age show sutures of great complexity. It is in these later forms — the ceratitoids and am- monitoids — that the illustrations of Agassiz's law of recapitulation are observed. If an adult am- monite shell, such as Tropites, be broken and the fragments lie arranged in order from apex to living-chamber, the sutures will be found to increase in complexity from simple curves to complex wavy lines. The simple suture will re- call that of the Silurian nautilus, the wavy sutiire that of the Devonian goniatite. and the complex suture is that of the normal ammonoi<l. Together with the change in the form of the suture-lines, there is a change in the position of the sipbuncle. This is illustrated in the sections of I'altopleuroeeras and Parkinsonia (Cephalo- ]<oda. Plate I.. Figs. 1 and 2). where in the first cliambcr the sipbuncle is central, as in Xautilus, and in successive chambers apprnnchci nc;ircr tn the outer ventral wall of the shell, with which it finally unites. We have here instances where the shell passes in its development from embryo to adult through stages that recall the forms of its past ancestors. Some of the more complex types of ammonitoid sutures are shown in figures 0, 0. and 10 of Plate I., where also the illustrations of Phylloceras (Figs. 10 and 11) >liow the rela- tions between the crimping of the septum and the complexity of the suture-line. The embryos of ammoiioids (Plate I., Figs. 14 to 19) exhibit three types distiiiguislied as asel- late, latisellate, and angustisellate, which terms refer to the form of the first suture-line. As ,a rule the asellate protoeonchs are the more primi- tive, as in the early goniatitoids. The later goniatitoids have latisellate embryos. The an- gustisellate embryos appear in the Triassic genera of ceratitoids, and nearly all of the am- monitoids of the .Turassic and Cretaceous have angustisellate jirotoconchs. .Many ammonoids are known to liave jiossessed oiiercular plates that served to close the .aperture of the shell after the retreat of the animal, as shown in the illus- tration of Opijclia (Plate L, Fig. 13). This organ is known as the anaptychus when single, and aptyclius when double, and examples of it were in early days supposed to be the carapaces of phyllopod crustaceans. In form, the ammonoids varv from globular shells (CycU)lobus, Plate II., Fig. 4) to flattened discoids ( Pinaeoceras, Plate II.. Figs. 10 and 11), and their surfaces are variously ornament- ed, from smooth (Tcpliyroceras (Plate II., Fig. 16), costate Dipoloceras (Plate II,, Fig. (i). to nodose Trachyceras (Plate II., Fig. 9), and in the phylogerontic (senile) stages of some races short spines are develo|)ed ( Ancvloceras, Plate 11.. Fig. 17). The normal ammonoid shell is a Hat. closely wound coil, but the latest members of several races in the Cretaceous rocks have the shell uncoiled or straight in the adult stages, though closely coiled in the early stages of their growth. In some the uncoiling begins only in old age (.Scaphites. Plate II.. Fig. 5. and Ma- croscaphites. Plate 11., Fig. 3) : in others it is accelerated and appears at relatively earlier periods in the development, as in Baculites (Plate 11., Fig. "), Spiroceras (Plate II., Fig. 1), Hamites (Plate II., Fig. 2), and Ancyloeeras (Plate II., Fig. 17). The uncoiling sometimes produces a tiirreted shell, as in Tiirrilites (Plate II., Fig. 1.51. These uncoiled shells of the Cre- taceous rocks are degenerate descendants from earlier normal Cretaceous or Jurassic ancestors, as can be demonstrated by the forms of their sutures. The degeneracy is exhibited also in the form of the sutures, which have become more simple, and in the shape of the cone, which ha.s lost its tlattencd section due to the closely coiled condition, and as-iimcd the rounded, cylindrical section of its earliest ancestors of the Paleozoic, though it retains the "impressed zone' to serve as evidence of its relationship. For notes on the fossil forms of dibranchiates, see articles re- ferred to at end of the bibliography. BiBLiOGRAi'iiY. For general description of the anatomy and habits of cephalopods, consult: Fischer, Maiiiiel dr roiwliiilinlnfiie el rfc pnlinn- tnlonie cnnchf/liolopifiue (Paris, 1887): Owen, Memoir on the Pnnli) vfilii.i (London. 1832) ; Willey, "In the Home of the Xautilus," in at- will keiriirr. Vol. VI. (London. 1805) : Moseley,