501 



S. with an opaque, white fhell, witli two volutions, 

 deeply grooved longitudinally, or in a fpiral direftion, 

 and tranfverfely wrinkled, efpecially in the furro^^'s: 

 aperture round. Size of the Spirorbis. 



This fpecies bears fome refemblance to S. lidcrojlro- 

 pha, but is at once diftinguiflied by the whirls not being 

 reverfed : the two fulci are much deeper, making the 

 upper part of the (hell more ftrongly tricarinated : the 

 inner volution is nearly obfolete, only being marked by 

 the third, or interior ridge. 



This is alfo much more rounded at the top, with only 

 an umbilicvs in the middle ; whereas the other is con- 

 cave at the top, and the interior volutions confpicuoufly 

 tapering to a fine point, within the depreflion. In co- 

 lour and fize too they alfo differ ; this is of a white, like 

 glafs enamel, but not fo glolfy ; whereas, the heterojlro- 

 pha is of a dull brownifli, or dirty-white, and never 

 grows to half the fize. 



We have found this fpecies in great abundance on the 

 rocks, as well as on the under fide of loofe (lones at 

 Milton ; and fometimes on old fliells from other parts of 

 the coaft of Devonjlnre, efpecially Area piLoJa ; and hav<^ 

 received it from Guemfey on HaUotis tubercutnta, fpe- 

 cimens of which we have feen perfeftly covered with it, 

 but fometimes in company with a fpecies of Serpul i very 

 .different from any of the genus hitherto defcribed. It 



has 



