ORDER PECTINIBRANCHIATA. 55 
little prominence or vestige of a tooth, similar to that of the 
monodontes, from which these trochi only differ in the angle 
of their aperture, and the projection of their margin. The 
aperture is usually about as high as itis wide. (CANTHARIS, 
Montf.; Troch. iris, Chemn., &c.) 
In some of them, on the contrary, the aperture is much 
wider than it is high, and their convex base approximates 
them to the calyptracea. (INFUNDIBULUM, Montf.; Tro- 
chus concavus, Chemn.) 
In others, where the aperture is also much wider than it is 
high, the columella forms a spiral canal. (Trochus foveola- 
tus, Chemn.) 
Those which have a turreted shell approach cerithium, 
(TELEScoPIUM, Montf. ; Trochus telescopium, Chemn.) 
Among the umbilicated trochi there are some in which 
there is no projection in the columella; most of them are 
flattened, and have the external angle trenchant. Of this 
number is 
Tr. agglutinans, L., Chemn. V. clxxii. 1688—9, remark- 
able for the habit of glueing to its shell, and even incorporat- 
ing with it, as fast as it increases in size, various foreign 
bodies, such as little pebbles, fragments of other shells, &c. 
It frequently covers its umbilicus with a testaceous plate. 
Add Troch. indicus, Chemn., &c. 
The margin of others, however, is rounded; such as 
Tr. cinerarius, L., Chemn. V. clxxi. 1686: asmall species, 
and the most common on the coast of France; greenish, ob- 
liquely streaked with violet. 
Other umbilicated trochi have a prominence near the bot- 
tom of the columella. (77. virgatus, Chemn., &c.) 
And finally, there are some in which it is longitudinally 
crenate. (Zr. maculatus, Chemn., &c.) 
