PROSOBRANCHIATA. 189 
tinguishes the Pleurotome; the condition of the outer lip, which is much thickened 
within, and so strongly arched as to be almost semicircular in form; the deep, wide 
sinus, which divides the posterior extremity of the outer lip from the suture, and 
exactly resembles the notch by which the Psewdotome (a section of the Pleurotome 
proposed by Bellardi) are distinguished ; and the elevated, reflected anterior margin of 
the columellar lip, forming the right wall of the anterior canal. 
The recent cones, distinguished by the beauty and variety of their colouring, are 
very numerous: three hundred and sixty-nine species are enumerated by Messrs. 
Henry and Arthur Adams in the different divisions adopted by those authors; and, 
excepting two species which are found in the Mediterranean, all are inhabitants of 
tropical seas, abounding chiefly in those of Asia. They inhabit fissures and holes 
in rocks, and coral reefs, ranging in depth from low-water mark to thirty or forty 
fathoms. 
In a fossil state, the genus first occurs in the upper cretaceous strata. M. 
Deslongchamps, it is true, has referred to it certain shells from the lias of Calvados, 
exactly resembling cones in outward form, and which, if the genus were correctly 
determined, would present the anomaly of the cones not being represented during the 
long epoch which elapsed between the deposit of the middle lias and that of the 
upper chalk. M. D’Orbigny, however, found, on examination, that the inner whorls 
were as thick as the outer ones; and from this circumstance he has inferred that the 
shells in question are not true cones, and he has referred them to Acteonina, a genus 
peculiar to the Oolitic formations, and proposed by him for certain Acteon-like shells, 
without teeth or folds on the columella. And thus the apparent anomaly disappears. 
In Europe two species only have hitherto been found in the Chalk—one from Tours, 
described by Dujardin; the other from Martigues (Bas du Rhone), described by 
Mathéron: and from the eocene strata, sixteen species have been described by 
Solander, Bruguiére, Lamarck, Sowerby, and Deshayes. After the eocene era, the 
genus disappears from our Fauna; although, on the Continent, it appears to have 
been largely developed during the miocene and pleiocene epochs, sixty-seven species 
having been described by Grateloup, Michelotti, Brocchi, Borson, and other authors, 
from the formations of those periods, in France, Italy, and Germany. In America 
only four species, I believe, have as yet been found—one in the Chalk of South 
Carolina, a second in the eocene strata of Alabama, and two in the more recent 
formations. 
bo 
or 
