F0RAMIN1FERA OF THE NORTH PACIFIC OCEAN. 



In general the genus Opthalmidium has the following development 

 a proloculum, followed by a close-coiled Comuspira chamber of two 

 or more coils in length, in turn followed by a series of chambers in 

 general about two-thirds of a coil in length and tending to uncoil at 

 their apertural ends, the space thus left being filled in by shell growth 

 as the following chamber is built, the adult character being a series 

 of chambers close coiled and one-half a coil in length. 



Development of Planispirina. — Planispirina is a genus not greatly 

 removed in some respects from Cornuspira and Opthalmidium. The 

 species P. exigua H. B. Brady will serve to show the development. 

 Figure 7 shows a specimen viewed by transmitted light. There is a 

 globose proloculum followed by a very long Cornuspira-\ike second 

 chamber consisting of two and a half coils of continuous tube. In 

 this long second chamber there is a relatively primitive development 

 compared to that seen in the second 

 chamber of Opthalmidium. The third 

 chamber makes a half revolution as 

 does also the fourth. This condition 

 is much like that seen in S piroloculina 

 or in later stages of Opthalmidium. 

 Planispirina differs from the young of 

 S piroloculina in its relatively less ac- 

 celerated second chamber and also in 

 its adult characters. It differs from 

 the young of Opthalmidium in slap- 

 ping the stage having chambers two- 

 thirds of a coil in length, a stage in- 

 termediate between the long chambers 

 of the early stages and the half coil 

 chambers of the adult in Opthalmidium. 

 The fifth chamber is shorter than the 

 preceding ones, in this specimen being about a third of a coil in length. 

 The sixth and seventh chambers complete the coil begun by the fifth 

 chamber and give the adult character, that of a test with three or four 

 chambers in each volution. 



From the exterior the chambers of the last coil are clearly visible, 

 but those of the interior are hidden, as there are no definite markings 

 on the surface to indicate the chambers. 



Development of S piroloculina. — The genus S piroloculina is much 

 more abundant in number of species and individuals in the Eocene 

 deposits of France than it is in the present ocean. It reached a high 

 development in the Eocene and a number of senescent forms are seen 

 in those formations, more than in the recent species which are com- 

 paratively few and simple. S piroloculina differs from its nearer 

 53709°— Bull. 71, pt. 6—17 2 



Fig. 7. — Planispirina exigua H. B. Bkadt. 

 (Adapted from Bbady.) Long second 

 cornuspiralike chamber Proloculum 

 (l). (2) followed by half con, length 

 (chambers 3, 4) AND LATER byone-tuird 

 COLL length(ciiambers 5-7). 



