151 



they have been imbedded. The number of spiral turns also varies con- 

 siderably in different specimens, and evidently according to the size and 

 growth of the body. Thus, in those of a quarter of an inch in diameter, 

 the number of whirls is from three to four; and in those of an inch in 

 diameter, the number is not less than two dozen. One of these bodies 

 being split transversely through the spiral, beginning in the centre, and 

 continued to the circumference, is seen on each of the inner faces. 

 These spiral turns are divided into numerous chambers, by transverse, 

 supposed imperforated septa, which are a little obliquely extended 

 towards the centre of each disk, and become gradually lost between the 

 approximated plates. An idea of this part of the structure is given 

 Plate X. Fig. 14. In feet, the external plate of each turn of this spiral 

 is folded in two, its particular chambers being formed in the angle of the 

 fold, which then is extended above and below, covering and uniting 

 itself with the outer sides of the inner plates. The specimen, Plate X. 

 Fig. 15, will point out this curious mode of structure. Little processes 

 may be seen going off* from the different fractured edges of the plates in 

 the centre, to form the septa in their correspondent whirls. 



This same specimen will illustrate a part of the structure which does 

 not appear to have been yet explained. On the surface of the middle 

 plate of this specimen, small undulating risings are observable : these, 

 by their attachments to the under side of the plate which laid over this, 

 connected the two plates together, leaving a vacant space between the 

 plates, where these risings did not exist. As these seeming irregularly 

 undulating lines approach the circumference of the plate, they assume 

 a greater regularity in their direction, and give off, at equal distances, 

 the septa dividing the chambers which are disposed round the folded 

 edge of the plates. 



In another species, Plate X. Fig. 17, this connection between the 

 upper and under side of the plates is formed by numerous small and 

 short columnar projections, the points of attachment of which may be 

 seen on both surfaces. 



