FORAMINIFEEA OF THE NOETH PACIFIC OCEAN. 91 



X' it is simply flattened. This is interesting in comparison with the 

 section of the actual two-chambered young of Pcneroplis figured by 

 Schako (fig. 1). where the wall of the proloculum is simply flattened 

 on the two sides and not bent inward. Lister's figure of a similar 

 section of 0. adunca shows this concavity at one side and flattening at 

 the other as in our figure. At A is shown the section across a chamber 

 in the position of chamber 8 or 9 in the other section (fig. 45) — 

 that is, a chamber at the point where one complete coil has been 

 made. At A' is shown a portion of a chamber on the opposite side. 

 These two chambers indicate in section the width of the test at the 

 second coil. Chambers B and B' show the thickness of the test at 

 the end of the third coil and C and C at the completion of the fourth 

 coil. Each coil slightly overlaps its preceding half, as is seen in the 

 cross section. After making about four complete coils the chambers 

 increase greatly in height in this form. This may continue until the 

 wing-like growth thus formed swings completely about the test, but 

 in this species the ends of a chamber do not unite with one another 

 to form annuli, but the growth turns back onto the earlier portion 

 of the test and at this edge the test becomes formed of two nearly 

 parallel plates. 



0. compressa typically forms but one and a half coils before begin- 

 ning to extend back on its own periphery and very soon thereafter 

 the opposite ends of a chamber unite and form a complete annulus. 

 The annuli are formed throughout further growth and the size of the 

 test increases rapidly. 



The two types of development in later stages are very different, 

 that of 0. compressa, the simpler, more nearly like Pcneroplis and 

 leading directly toward Orbitolites, especially the simplest species 

 0. marginalis which it resembles in many respects. The later devel- 

 opment of 0. adunca, however, is more complex and tends in certain 

 respects toward the genus Alveolina. 



ORBICULINA ADUNCA (Fichtel and Moll). 



Plate 37, figs. 7, 8. 



Nautilus aduncus Fichtel and Moll, Test. Micr., 1803, p. 115, pi. 23. 



Orbieulina adunca Lamarck, Tabl. Encycl. et Moth., 1816, pi. 468, figs. 2a-c. — 

 d'Orbigny, in De la Saga, Hist. Fis. Pol. Nat. Cuba (Foraminiferes) 1839, 

 p. 81, pi. 8, figs. 8-14. — H. B. Brady, Rep. Voy. Challenger, Zoology, vol. 9, 

 1884, p. 209, pi. 14, figs. 1-13. 



Description. — Test typically planospiral, involute, each coil com- 

 pletely covering the previous one; chambers increasing rapidly in 

 height, much curved; sutures distinct, but slightly depressed; wall 

 smooth or slightly pitted; aperture consisting of usually a double 

 series of small rounded pores, each with a slight raised border, on 

 the apertural face of the test, which is flattened or slightly depressed. 



