FOEAMIlsriFEEA OF THE ATLANTIC OCEAN 77 



Family 17. OPHTHALMIDIIDAE 



Test calcareous, imperforate, early chambers at least planispiral, 

 except in degenerate forms; wall without an arenaceous coating; aper- 

 ture typically open, without a tooth. 



This family is somewhat closely related to the Miliolidae and yet 

 the two have few points in common. The early history of the Oph- 

 thalmidiidae is now much better known than formerly due to the 

 greater knowledge of the Carboniferous and Permian foraminifera. 

 The ancestry of the group can be traced back to Glomospira-like 

 beginnings and the development of the calcareous, imperforate test 

 from such arenaceous forms. Cornuspira itself while very primitive 

 is probably derived through such forms as Hemigordius . In Hemi- 

 gordius, the early stages, at least in the microspheric form, are coiled 

 in varying planes finally becoming planispiral in the adult. Hemi- 

 gordius is only known from the Carboniferous. By acceleration of 

 development, these early stages become progressively reduced and 

 the planispiral Cornuspira results. The very early forms of Cornus- 

 pira are often seen with the earliest coils irregular, but in later geo- 

 logic periods the planispiral character becomes fixed in both the 

 microspheric and megalospheric forms. 



In Vidalina, in the Cretaceous, large forms are developed which are 

 completely involute but such forms were specialized and did not per- 

 sist beyond the Cretaceous. In Cornuspiroides the height of the 

 coil greatly increases and a fan-shaped test results. These are known 

 only from the present ocean. In Cornuspirella the later stages are 

 very greatly spreading into long, branching, peripheral expansions, 

 representing also a specialized type of the present ocean. 



The next stage in development is for the undivided chamber to be 

 divided into definite chambers. These may be in a more or less recti- 

 linear series as in Nodohacularia, or be attached and spreading in 

 different directions as in Cornuspiramia . Nodohacularia developed 

 at least in the early Jurassic and still persists. 



In the next group of genera the coiled chambers become divided 

 into chambers growing progressively shorter as the test is developed. 

 Such simple forms as OpJithalmidium are known from the early Juras- 

 sic as is also Spiropihalmidium, and both persist to the present ocean . 

 Planispirina developed at least as early as the Cretaceous. Renulina 

 became specialized in the Eocene, the chambers elongating, finally 

 becoming annular and the genus not persisting beyond the Eocene. 

 The most complex form of all, Disco spirinia, is one of the best of the 

 foraminifera to show developmental stages. As it is but a single 

 chamber in thickness and the wall very thin, the whole test may be 

 viewed by transmitted light. The early chambers follow the usual 



