FAMILY 17. OPTHALMIDIIDAE 159 



The Miliolidae form a very complete and continuous series 

 from the undivided Agathamynina of the Carboniferous to the 

 very complex Lacazina of the Upper Cretaceous. The micro- 

 spheric form in section shows the various developmental stages 

 and their sequence whereas they are often skipped in the 

 acceleration of the megalospheric form. The climax of the 

 family was reached in the Upper Cretaceous where, in various 

 parts of the world as well as in the Eocene, "Milioline" limestones 

 were formed. In the present oceans the Miliolidae are abundant 

 in warm shallow waters, but are represented by simpler forms 

 than those developed in the Upper Cretaceous. The most com- 

 plex forms now living are found in the Indo-Pacific. A few 

 genera, notably Pyrgo and Sigmmlina, have in some species 

 become adapted to deeper and colder waters. Under deep sea 

 conditions or in brackish water, siliceous and chitinous tests 

 may be formed showing the relationships to the Silicinidae and 

 the earlier chitinous groups. The habitat of incorporating sand 

 grains in the outer layer of the test in the primitive form of 

 the Miliolidae also indicates their close relationships to the 

 arenaceous group. 



FAMILY 17. OPHTHALMIDIIDAE 



Test calcareous, imperforate; early chambers at least plani- 

 spiral, except in degenerate forms; wall without an arenaceous 

 coating; aperture typically open, without a tooth. 



KEY TO THE GENERA 

 Test free. 



A. Early chambers at least planispiral, undivided, no chambers a half 

 coil in lengrth. 



1. Test of proloculum and long undivided tubular chamber, close 



coiled. 

 a. Planispiral throughout. 



(1) . E volute Cornuspira. 



(2) . Involute Vidalina. 



h. Early stages not entirely planispiral Hemigordins. 



2. Test in the adult fan-shaped, becoming bilaterally symmetrical. 



Comuspiroides. 



