8i6 



MICROSCOPIC FORMS OF ANIMAL LIFE 



shells, both of the ' poi-cellanous ' and the ' vitreous' scries : and the 

 most remarkable of these is the Cyclwrwmina concellatn (fig. 616), 

 which has been brought up in considerable abundance from depths 

 ranging downwards to 1.900 fathoms, the largest examples being- 

 found within 700 fathoms. The test (fig. 616, a) is composed of 

 aggregated sand-grains firmly cemented together and smoothed over 

 externally with ' plaster.' in which large glistening sand-grains are 

 sometimes set at regular intervals, as if for ornament. On laying 

 open the spire it is found to be very regularly divided into chambers 

 by partitions formed of cemented sand-grains (b). a communication 

 l>et \veen these chambers being left by a fissure at the inner margin 

 of the spire, as in Operculina (fig. 628). One of the most curious 

 features in the structure of this type is the extension of the cavity 

 of each chandler into passages excavated in its thick external wall, 



FIG. 616. Gyclammina ctnirrl/nfti, showing at a, its external aspect ; 

 b, its internal structure ; c, a portion of its outer wall in section, more 

 highly magnified, showing the sand-grains of which it is built up and 

 the passages excavated in its substance. 



each passage being surrounded by a very regular arrangement of 

 sand-grains, as shown at c. It not ^infrequently happens that the 

 outer layer of the test is worn away, and the ends of the passages 

 then show themselves as pores upon its surface ; this appearance, 

 however, is abnormal, the pas>ages simply running from the chamber- 

 cavity into the thickness of its wall, and having (so long as this is 

 complete) no external opening. This labyrmthic- ' structure is of 

 great interest, from it.v relation not only to the similar structure 

 of the large fossil examples of the same type, but also to that which 

 is presented in other gigantic fossil arenaceous forms to he presently 

 described. 



Although some of the nautiloid Lilnnln -are among the largest 

 of existing Koraminifera. having a, diameter of fK! inch, they are 

 mere dwarfs in comparison with two gigantic fossil forms, whose 



