A manual of palaeontology for the use of students : with a general introduction on the principles of palaeontology / by Henry Alleyne Nicholson and Richard Lydekker.

Date:: 1889

Credit: A manual of palaeontology for the use of students : with a general introduction on the principles of palaeontology / by Henry Alleyne Nicholson and Richard Lydekker. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

867/1688 page 845

$the siphuncle is variable in position, but is usually placed between the centre and the convex side] and the surface is commonly adorned with tubercles or ribs. The genus ranges from the Silurian to the Carboniferous. In Hercoceras the shell is discoidal, with contiguous whorls and a wide um- bilicus. The aperture is contracted ; the sutures are simple \ the siphuncle is sub- marginal and external; and the surface is adorned with transverse strise or a row of projecting tubercles. The two known species are Silurian. In the genus Nautihis itself (fig. 759) the shell is involute or discoidal, consisting of a few whorls coiled into a flat spiral, the volutions being in contact, and the last turn of the shell commonly more or less concealing the previous ones. In the young condition of the shell a central vacuity exists behind the conical initial chamber; but in the typical forms of the genus this becomes ultimately filled up by a secondary deposit of shell, or becomes concealed by the later volutions. The body-chamber is capacious, and the aperture is simple and has a ventral sinus. The Fig. 758.—Gyroceras omaium. Devonian. J' 'g- 759-—Nautilus DanicHS. Upper Cretaceous. septa are concave forwards, and the suture-lines are simple or may show a slight ventral or dorsal lobe. The siphuncle is subcen- tral in position, or may be placed between the centre and either the ventral or the dorsal margin, and it is never contracted by internal deposits; while the septal necks are always short. The surface may be quite smooth {Lavigati\ or adorned with transverse striae (Striati), or with markedly distinct ribs {Radiaii). The genus Nautilus has been split up into a number of minor$