MOLLUSCA. 115 
inhabitant been satisfactorily enlarged. The Dentalium im- 
perforatum, trachea, and glabrum of Montagu’s Testacea 
Britannica, do not accord with the essential character of 
the Linnzean genus in being “utraque extremitate pervia.” 
33. SeRPuLA.—This genus has undergone several changes 
in the hands of modern conchologists. The \S. seminulum 
has been transferred to the genus miliola, and the S. filo- 
granum to the tubipora. Besides these trivial alterations, 
the character has been greatly circumscribed, so as only to 
include shells which adhere to other bodies, and are tubu- 
lar, entire and flexuous, with a simple mouth, as represent- 
ed by the S. contortuplicata of Linnzus. The species 
which are regularly spiral, discoid, and fixed, as the S. spi- 
rorbis, now constitute the genus Sprrorsis. But as there 
are both dextral and sinistral shells with this character, 
the dextral species may form the genus Spirorbis, while 
the HeTEROpISCA may receive the reversed species. 
The genus VermicuLaARria is formed from those species 
which, in appearance, resemble the spirorbis, but are not 
adherent, such as the S. lumbricalis. The shell at the 
mouth is, in general, somewhat produced. ‘There are two 
or three minute British shells of this genus. 
The genus Srz1quaRriA, represented by the S. anguina, 
is distinguished from the serpula by a longitudinal, lateral, 
subarticulated fissure, which extends the whole length of 
the shell. 
The genus Pentcriuvs is formed from that curious shell 
the |S. penis, and well known by the name of the watering- 
pot. The disk is perforated by a number of small holes. 
34. TerEpo.—From this genus, now considered as a bi- 
valve (the tube being regarded as an accessory covering), 
the Fistunana, of which T. clava of Gmelin is the repre- 
