A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . Fig. 40. Gomphoceras. Fig. 41. Phragmoceras* M. Barrande, from whose great work on the Silurian Formations ofBohemia these figm-es are taken, suggests that the lower part of the aperture{s s) which is almost isolated, may have served for the passage of the funnel,whilst the upper and larger space [c c) was occupied by the neck ; the lobesprobably indicate the position of the external anns. The apertui-e of the pearly nautilus is closed by a disk or hood (fig. 43, h),formed by the union of the two dorsal arms, wh

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A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . Fig. 40. Gomphoceras. Fig. 41. Phragmoceras* M. Barrande, from whose great work on the Silurian Formations ofBohemia these figm-es are taken, suggests that the lower part of the aperture{s s) which is almost isolated, may have served for the passage of the funnel,whilst the upper and larger space [c c) was occupied by the neck ; the lobesprobably indicate the position of the external anns. The apertui-e of the pearly nautilus is closed by a disk or hood (fig. 43, h),formed by the union of the two dorsal arms, which correspond to the sheU-secreting sails of the argonaut. In the extinct ammonites we have evidence thatthe aperture was guarded stiU more efiectively by ahomy, or shelly operculum, secreted, in all probabi-lity, by these dorsal arms. In one group (arietes,)the operculum consists of a single piece, and is hornyand flexible.! Iji the round-backed ammonites theoperculum is sheUy, and divided into two plates by astraight median suture (fig. 42). They were de-scribed in 1811, by

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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmollusks, bookyear185