163 



spine, (fig. 1, a). In a still younger state, these are prolonged into 

 stellate marginal rays (fig. 2, a). 



The calcareous shell is divided into a great number of radiating 

 compartments ; the line of division being often marked externally by 

 an elevated ridge. The interior of each chamber is convex pos- 

 teriorly,* and concave anteriorly ; on the exterior of each of these 

 subdivisions, the shell is marked by a series of transverse folds or un- 

 dulations (fig. 1, b, c), the "fossettes" of D'Orbigny, which are very dis- 

 tinct near the convex septum of the preceding cell, but usually 

 disappear before reaching the concave margin of the subsequently 

 formed chamber. The front wall of the last chamber (fig. 1, d) 

 is nearly smooth ; the central portion being usually flattened or trun- 

 cate. Its outer margin exhibits no traces of large oral (?) orifices, 

 communicating with the interior of the first chamber, but where the 

 septum is in contact with the preceding convolution, there is an 

 arched row of very small, and often wholly invisible apertures. 

 These are usually so indistinct, that I long sought them in vain, and 

 only from a few specimens obtained satisfactory evidence of their 

 existence. As already remarked, the periphery of the young or- 

 ganism, when obtained from deep water, instead of from the water- 

 worn sands of our beaches, is furnished with a circle of large, radiat- 

 ing, transparent, calcareous spines ; those connected with the earliest 

 formed cells being proportionally the longest (fig. 2, a). Whatever 

 may be their use, they appear to become less necessary to the economy 

 of the animal in its matured state, since in the larger and more re- 

 cently formed segments, they first diminish in size, then degene- 

 rate into mere tubercles, and ultimately are wholly wanting (fig. \,a). 

 They are not, however, invariably present, even in the youngest indi- 

 viduals ; some of the latter occurring not unfrequently with an entire 

 periphery, which does not present even the slightest vestige of a 

 spine. In this inconstancy they resemble many other forms of 

 Foraminifera. The spine is to the single cell of a Polystomella, 

 what the umbo is to a Lagena and an Entosolemia, or to the mucro 

 of a Triloculina and a Nodosaria. It is an uncertain characteristic, 

 which may be present or absent without affecting the integrity of the 

 species. When by the growth of the organism these species become 

 enclosed within the cells of the surrounding convolutions, they 



* I have used the term " anterior,'' when referring to the convex external margin 

 of each cell, and " posterior," when to the opposite or concave border. 



