WHELKS, ETC. 185 
becoming thinner and more convex. He found 
that so long a time as four months elapsed before 
the vesicle opened, and then the included whelklings 
did not quit their cradle all at once, but took their 
time in coming out, according to their individual 
dispositions ; doubtless, the quick-minded and more 
curious commencing their travels first, whilst those 
of slow and studious constitutions would remain 
as long as a fortnight before resolving to see the 
world, which with young Purpurew is no very 
dangerous adventure, since the neighbouring bar- 
nacles enable them to look about with safety, 
before making a long journey from their birth- 
place.* 
FAMILY VELUTINIDA. 
A small and unimportant group is indicated by 
this name, represented in Britain by two genera, 
each consisting of two species. ‘They have a shell, 
the aperture of which is very broad and open, and 
the spire minute ; in texture it is thin, sometimes 
pellucid, and sometimes even membranaceous. In 
one genus it is entirely included within the sub- 
stance of the mantle, as in Pleurobranchus: in the 
other it is external, but partially invested by the 
edges of the mantle, and covered with a skin 
(epidermis). 
The animal is large, with a short broad head, 
furnished with two tentacles, and eyes at the 
exterior of their bases. ‘There are two gill-plumes. 
The operculum is wanting. 
Our most common species is Velutina laevigata, 
reckoned by Linnzeus among the snails, and long 
supposed to be a fresh-water mollusk ; it is, how- 
* Cited in Forbes and Hanley, iii. 384. 
