194 



TWENTIETH REPORT ON THE STATE CABINET. 



(Plate i, figs. 1 ,2, and 3) ; while in one of the bi-celluliferous species 

 from Iowa, the cellules are distinct from each other at their origin, and the 

 upper extremity of one scarcely reaches to the base of the next in advance 

 (Plate ii, fig. 10), and they are therefore not properly in contact in any 

 part of their length. The same is more emphatically true of Rastrites 

 (fig. 29), where there is a large interval between the bases of the cellules, 

 which are often nearly rectangular to the axis. 



The earlier or first developed cellules are always smaller than those fol- 

 lowing, and they reach their full development at difl'erent distances from 

 the base or axil, in difl'erent species. Toward the distal extremities the 

 cellules usually become gradually smaller, to the last one, while some- 

 times they become abruptly, less and less; the latter feature probably 

 occurring in young and vigorous growing individuals, while the gradual 

 diminution is apparently characteristic of an approach to maturity. 



Fig. 30. 



:> 



Rastrites barrandi, 

 natural size, with a portion enlarged. The cellules consist of long slender tubes. 



Although we regard the cellule as limited by the cell-partitions, yet in 

 well-preserved specimens there is sometimes a swelling of the test of the 

 common body below the cellule, indicating an enlargement of the parts at 

 the bases of the polypites. In one species there is an evident undulation of the 

 axis, corresponding to this enlargement of the parts in the common body 

 (Figs. 10 and 11, Plate ii). 



In the diprionidian species, the cellules on the two sides of the stipe 

 are alternating, so that the bases or the apertures are opposite the space 

 between two others. This is more especially shown in the enlarged figures 

 10 and 12, Plate ii. 



In much the larger proportion of species, the body of the graptolite and 

 the cellules are so extremely compressed, that they appear only as serra- 

 tures along the margin, with distinct impressed lines marking the 

 cell-divisions. The exterior margin of these serratures indicates in an 

 approximate degree the outline of the aperture ; and the frequently occur- 



