8i6 



MICROSCOPIC FORMS OF ANIMAL LIFE 



shells, both of the * porcellanous ' and the ' vitreous ' series ; and the 

 most remarkable of these is the Cyclammina cancellata (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 (6), a communication 

 between 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 chamber into passages excavated in its thick external wall, 



FIG. 616. Cyclammina cancellata, showing at a, its external aspect ; 

 &, 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 unfrequently 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 passages simply running from the chamber- 

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

 complete) no external opening. This * labyrinthic ' structure is of 

 great interest, from its 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 be presently 

 described. 



Although some of the nautiloid Lituolce are among the largest 

 of existing Foraminifera, having a diameter of 0'3 inch, they are 

 mere dwarfs in comparison with two gigantic fossil forms, whose 



