Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 94 PARTS OF UNIVALVE SHELLS. Base. plane, but more usually in an oblique direction. As a general rule, the spiral univalve, if viewed in the position in which its inhabitant would carry it, if moving forwards from the observer, is twisted, from the apex downwards, from left to right, the spire being directed obliquely to- wards the right. The annexed figure {fg. 116) shows the involutions or whorls of the spire of the Pleurotoma. In some genera, for example,
Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 94 PARTS OF UNIVALVE SHELLS. Base. plane, but more usually in an oblique direction. As a general rule, the spiral univalve, if viewed in the position in which its inhabitant would carry it, if moving forwards from the observer, is twisted, from the apex downwards, from left to right, the spire being directed obliquely to- wards the right. The annexed figure {fg. 116) shows the involutions or whorls of the spire of the Pleurotoma. In some genera, for example, Clausilia (from the Latin, clausvs, s\\\it,fg. 25, page 40) and Phijsa (from the Greek, â phusa, a bubble,^^. 30, jmge 42), the shell is twisted in an opposite direc- tion : such shells are called ' per- verse,' or ' sinistral.' The aperture or mouth, is that part of the lower whorl or body through which the animal protrudes itself. This is one of the principal means of distinguishing the genera of univalve shells, and it varies much in its form; some apertures are rounded {fgs. 43 and 46); others semilunar (Jig' 65); others angular {fig. 37), &c. Some apertures have a canal at their base {Jigs. 63 and 116), and others are with- out it {Jig. 51). In various genera, the aperture extends the whole length of the shell, as in Ovula {Jig. 68), Cyprasa, and some of the Cones with depressed spires {Jig. 70). In several individuals, the aperture is either entirely open, or closed by an operculum or door, which is usually affixed to the foot of the animal. When without a notch or canal, the aperture is said to be entire {fig. 51). The aperture has two lips or borders; the internal lip, or border^ is on the side of the aperture, formed by the columella, and the , or outer lip, or border, is oppo- site, as in the Pleurotoma {fig. 116) ; the letter b. indicates the pallial notch; n. notch of the syphon. Canal, or gutter {fig. 116), is the space or hollow, formed by the prolongat
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