152 
Considering that the specific differences of these bodies are chiefly 
to be found in the markings of their surface and their internal struc- 
ture, I shall pursue my examination of these two circumstances, with 
a view to the necessary distinction of species ; and, since Lamarck 
divides this genus into the four following species, N. laevigata, 
N. glohularia, N. scabra, iV. complanata, which he states have been 
found in the environs of Paris, I shall attend to these circumstances, 
with reference to the specific distinctions of Lamarck. 
N. laevigata. Lenticular, smooth, and convex, on both sides. 
Plate X. Fig. 13. These, he observes, are found of all sizes, from 
that of a lentil to that of a twelve-sous piece. With the hope of 
acquiring some knowledge respecting the economy of this animal, or 
at least of the structure of its concamerated appendage, I rubbed 
down several specimens in a perpendicular direction, and gave to the 
sections as high a polish as I was able. By this operation a fair view 
was obtained of the concentric parietes of the chambers, with the 
short perpendicular septa by which they were divided. By this 
section it was also seen, that the processes connecting the different 
stories of chambers were placed immediately opposite to each other, 
forming short straight perpendicular lines between the upper and 
under surfaces. Plate X. Fig. 1(5. The facility with which these 
bodies are separated horizontally, also yielded me several opportunities 
of observing their internal structure, as represented Plate X. Fig. 14. 
It is there seen that the septa, placed in the spiral whirls, are dis- 
posed at nearly equal distances ; but the septa of one whirl maintain 
no particular order of position with those of the others. 
In every mass, almost, which I have seen, of this species, I have 
been struck with the circumstance of several of these bodies being 
split horizontally through their centre, so as plainly to shew the spiral 
turns continued to the very centre, as represented Fig. 14. I have 
one detached specimen, thus split, and showing the correspondent 
whirls and septa on each internal face. 
