A conchological manual . pex requires particularnotice, as the character of the whole shell frequently dependsupon the particulars observable in this part. It is sometimesobtuse, or blunt; sometimes acute, or sharp. In the Cones itis frequently flat, and in Planorbis it is concave. It is some-times of a different structure from the rest of the shell, retain-ing the horny and transparent appearance which characterizedit when the animal was first hatched. The Tritons presentan instance of this, although it is not always observable, owingto the tenderness of the substances which causes it to brea
A conchological manual . pex requires particularnotice, as the character of the whole shell frequently dependsupon the particulars observable in this part. It is sometimesobtuse, or blunt; sometimes acute, or sharp. In the Cones itis frequently flat, and in Planorbis it is concave. It is some-times of a different structure from the rest of the shell, retain-ing the horny and transparent appearance which characterizedit when the animal was first hatched. The Tritons presentan instance of this, although it is not always observable, owingto the tenderness of the substances which causes it to break orfall away in many specimens. A very remarkable instancealso occurs in Bulinus decollatus (cut, fig. 27, 28), so named,because the apex, to the depth of several whorls, falls off, andthe shell is decollated. In this, and many more instances,among Pupseform land shells, the occurrence of this circum-stance seems to be by no means rare or accidental, a provisionhaving been made for filling up the opeiiing by a septum. A. 3/ Jill ^^ Tig. 25, obtuse; 26, acute; 21, 28, decollated; 29, concave; 50,papil-lary; Q\,mamniellated; 32, discoidal. 22 INTRODUCTION. papillary apex is one which is swelled at the extremity into alittle rounded nob, or nipple; and a mammellated apex is onewhich is rounded out more fully into the shape of a teat. spire is described as consisting of numerous or fewwhorls, and sometimes the number of them is particularlystated. A whorl consists of one turn of the spiral cone. Thewhorls are described as flattened^ when the sides are notbulged out so as to cause the outline of the spire to deviateconsiderably from straightness: when the contrary is the case,the whorls are said to be ventricose, and either rounded orangidated. The degree of rapidity with which the whorlsbecome enlarged presents an important source of distinc-tion. The suture, or seam, which separates qrie whorl fromanother is also noticed as being distinct ox otherwise;canaliculated, or
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Keywords: ., bookcentury1800, bookdecade1840, bookidconchol, booksubjectshells
