152 



Considering that the specific differences of these bodies are chiefly 

 to be found in the markings of their surface and their internal structure, 

 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. l&vigata, N. globularia, N. sea- 

 bra, and N. 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. l&vigata. 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 struc- 

 ture 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 con- 

 centric 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 imme- 

 diately opposite to each other, forming short straight perpendicular 

 lines between the upper and under surfaces. Plate X. Fig. 16. 

 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 disposed 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 show the spiral turns 

 continued to the very centre, as represented Fig. 14. I have one de- 

 tached specimen, thus split, and showing the correspondent whirls and 

 septa on each internal face. 



