197 
SERPULA. Linn . 
Gen. Char.* Shell tubular, variously curved, fixed 
by a considerable portion of one side ; opercu- 
lum corneous or shelly, pedunculated, attached 
to the anterior extremity of an articulated, 
acephalous animal, whose feathered branchiae 
surround its mouth. 
The difficulty there always must be in distinguishing the 
four genera of Annelides that have shelly tubes, when we 
only have the means of examining the tubes deprived of 
their animals, has induced me to relinquish the attempt ; 
neither does it appear to be of much importance, since the 
animals are very similar, and their habits the same. 
The following are the four genera alluded to, which 
are distinguished by the characters attached. They may 
form subgenera. 
Spirorbis. Shell curved into a discoid volute, attached 
by its lower flattened surface. 
Animal furnished with six-pinnated branchiae and 
a peltate operculum. 
Obs. The shells are mostly minute, often produced 
beyond the convoluted part, and then irregularly 
curved. 
Scrpula . Shell irregularly contorted, fixed by a part of 
its side ; aperture simple. 
* The generic character given with tab. 30. not being suffi- 
cient, we add the present. 
