MONOGRAPH OF THE GENUS 

 CYCLOSTOMA, 



BY 



G. B. SOWERBY, F.L.S. &c. 



Char. Gen. Molluscum terrestre gasteropodum, pulmonic 

 ferum, sexibus separatis. Testa tvirbinata, polymorpha, spirae 

 apice obtuso ; anfractibus plerumque rotundatis : apertura 

 plerumque fere circular!, plus minusve post ice angulato ; 

 peritremate continuo, Scepe incrassato, reflexo. Operculum 

 pauci-vel vjaulti-spirali, calcareo vel corneo. 



The diversity of form of the shells composing this genus is 

 very remarkable, as will be seen by reference to our plates, 

 some of the species being turreted, others planorbular, with 

 every intermediate gradation ; the apex of the spire is always 

 obtuse ; the volutions are almost constantly rounded, and the 

 aperture is for the most part nearly if not quite circular, in 

 some instances somewhat elliptical and more or less angular 

 posteriorly ; the peritreme is generally continuous, sometimes 

 sinuated on the umbilical side, sometimes interrvipted at its 

 junction with the last volution, and sometimes notched 

 posteriorly, frequently simple and sharp-edged, and in many 

 instances reflected, or thickened and reflected, and variously 

 fringed, sometimes double, having an inner simple and an 

 external reflected edge. 



The recent species of this Genus are now very numerous ; 

 in 1822 Lamarck had only described 26 ; in 1832 I had drawn 

 up the characters of 108 species, and at the present time we 

 have represented and described 1 75 species. There are how- 

 ever still several of Lamarck's species Avith which we are not 

 acquainted, and of which we here mention the nam.es, in hope 

 to gain the information of which we are now dcitiiute : these 

 are his C. ambigua, C. decussata, C. lineolata, and C. tmiltilabris, 

 the number being now reduced to ybw;*, Avhereas in 1831 we 

 gave a list of six species which we then did not know ; we are 

 indebted to M. De Lessert for our present knowledge of 

 Lamarck's C. mamillaris and C. Orbdla ; the first of which 

 is identical with that called C, Yolziana, l)y Michfiud, 



