OCR Text |
Show 314 MOLLUSCA. much more various, as to form, than those now taken in the ocean( I). Chambered shells are also found among fossils, furnished with simple septa and a siphon, the body of which, at first arcuated or even spirally convoluted, remains straight in the more recent parts; they are the LITuus of Breyn, in which the whorls are sometimes contiguous(2), and sometimes distinct-the HoRTOLEs of Montfort. In others, the 0RTHOOERATITEs(3), it is altogether straight. Itis not improbable that the animal resembled that of the Nautilus or of the Spirula. The BELEMNITES Probably belong to this family, but it is impossible to ascertain the fact, as they are only found among fossils; every thing, however, proves them to have been internal shells, thin and double, that is, composed of two cones united at base, the inner one much shorter than the other, and divided into chambers hy parallel septa, which are concave on the side next to the base. A siphon extends from the summit of the external cone to that of the internal one, and con· tinues thence, sometimes along the margin of the septa and some· times through their centre. The interval between the two testaceous cones is filled with a solid substance here composed of radia· ting fibres, and there of self·involving conical layers, the base of each being on the margin of one of the septa of the inner cone. In one specimen we only find this hard portion, and in another we also find the nuclei of the chambers of the inner cone, or what are termed the alveoli. Most commonly these nuclei and the chambers themselves have left no other traces than some projecting circles on the inside of the internal cone. In other specimens again we find more or fewer of the nuclei, and still in piles, but detached from the double conical sheath that enveloped them. Of all fossils the Belemnites are the most abundant, particularly in chalk and compact limestone( 4 ). (1) Large species, with a single siphon; the ANGULITE, Mont., f. 1, 6;-the AG.A.NIDE, Id., 50;-the C.A.NTnon, Id., 46. (2) Nautiluslituus, Gm. ;-Naut. semilituus, Plane., I, :x. (3) Breyn. de Polythal., pl. iii, iv, v, and vi; and Walch, Petrif. of Knorr., Supp. IV, b, iv, d, iv. See also Sage, Journ. de Phys. an. IX, pl. 1, underthe name of Belemnite. ( 4) The best works on this singular genus of Fossils are the Memoiru NT Ia Belemnite& consideree& zoologiquement et geologiquement, by M. de Blainville, Paris, 4to, 1827; and that of M. J. S. Miller on the same subject in the Geol. Trans., se· eond series, vol. II, part1, London, 1826. See also Sage, Journ. de Phys. an. IX, and 315 M. de Blainville divides them according to the greater or less depth to which the internal cone or chambered portion penetrates, or as the edges of the external cone have a small fissure or not, or as the external surface is marked on one side by a longitudinal furrow, or by two or more furrows towards the summit, or finally as that surface is smooth and without furrows. Bodies very similar to Belemnites, but without a cavity and with a rather prominent base, form the genus AaTINOOAMAx of Miller. It is also upon conjectures of a similar nature that reposes the classification of the AMMONITEs, Brug. Or the Cornua-Ammoni( 1 ), for they no longer exist except among fossils. They are distinguished from theN autili by their septa, which, instead of being plane or simply concave, are angular and sometimes undulated, but most frequently slashed on the edge like the leaf of an acanthus. The smallness of their last cell seems to indicate that like the Spirula they were internal shells. They are very abundant in the strata of secondary mountains, where they are found varying from the size of a lentil to that of a coach wheel. Their subdivisions are based upon the variation of their volutes and siphons. The name of AMMONITEs, Lam., (SIMPLEGADEs, Montf., 82) is particularly restricted to those species in which all the whorls are visible. Theit• siphon is near the margin(2). They have lately been divided into the AMMONITEs, PLANITES of Haan, where the edge of the septa is foliaceous, and into the CERATirEs of Haan, where it is simply angular and undulated. Those in which the last whorl envelopes all the others form the 0RBITULITEs, Lam., or the GLOBITEs, and GoNIATITES of Haan, or the PELAGUSEs, Montf., 62, in all of which the siphon is situated as in the preceding ones. Haspail, Journ. des Sc. d'Observ., second No. To this genus we refer the Paclite Montf., I, 318;-the Thalamule, 322;-the .!lcheloi"te, 358;-the Cetocine, 370;-the /J.came, 374;-the Belemnite, 382;-the Hibolite, 386;-the Prorodrague,390;-the Pirgopole, 394, which are the cases of different spec;;_ies. As to the .!lmimone, Id., 326;-the Callirhoe, 362;-the Ch1·isaore, 378, they appear to be mere nuclei or piles of alveoli detached from their cases. (1) From this resemblance of their volutes to those of a ram's horn. (2) The various species of Ammonites have long been collected and described, but with less care than those of other shells. We may commence studying them in the article .!lmmonite, ;Ency. Method. Vers ., I, 28, and in that of M. de Roissy, in Sonini's Buffon, Mollusca, V, 16. See also the Monograph of Haan, entitled "Monographire .O.mmoniteorum at Goniateorurn Specimen," Leid., 1325. \ |