FORAMINIFERA OF THE ATLANTIC OCEAN. 



23 



to be ^^^dely distributed in the tropical regions. According to 

 Heron-Allen and Earland it is abundant off the British coasts. 



Williamson's figure named T. cuneiformifi, var. conica, resembles this 

 species in general but is apparently not the same as the tropical form. 



Textularia conica — material examined. 



Genus BIGENERINA d'Orblgny, 1826. 



Bigenerina d'Orbigny (type, B. nodosaria d'Orbigny), Ann. Sci. Nat., vol. 7, 

 1826, p. 261.— H. B. Brady, Rep. Voy. Challenger, Zoology, vol. 9, 1884, p. 

 368.— Chapman, The Foraminifera, 1902, p. 168.— Cushman, Bull. 71, U. S. 

 Nat. Mus., pt. 2, 1911, p. 27. 



Gemmulina d'Orbigny, Ann. Sci. Nat., vol. 7, 1826, p. 262. 



Description. — Test free, generally elongate, cylindrical or com- 

 pressed, the early portion textularian, composed of a series of biserially 

 arranged chambers, later chambers arranged in a single line; wall 

 usually thick, arenaceous, usually coarse but often smoothly finished ; 

 aperture in the young at the base of the inner margin of the last-formed 

 chamber, as in Textularia, but in the adult, in the uniserial portion ter- 

 minal, rounded or oval according to the form of the chamber. 



Both miscroplieric and megalospheric forms occur in the various 

 species. In the microspheric form as in the same form in Textularia 

 the earliest chambers may be arranged in a planospiral manner. In 

 the megalospheric form the coiled chambers are usually wanting and 

 the test starts with the biserial arrangement. As is usual with other 

 foraminifera the microspheric form of the species attains a larger size 

 in the adult than does the megalospheric 



From its developmental stages it is very evident that Bigenerina 

 represents a series derived from Spiroplecta through Textularia, and 

 is very closely allied to the latter. There are various stages in the 

 acceleration of development shown by various species. In some 

 the biserial series makes up the larger part of the test, the uniserial 

 series being only in the last-formed portion. In others the biserial 

 series is confined to the very early development and the larger part 

 of the test is uniserial. In the former cases specimens are often found 

 which have not yet reached the adult uniserial condition, and would be 

 taken for a species of Textularia were it not for their association with 

 adults with the full characters of Bigenerina. Such specimens occur 

 in the West Indies, especially those oi Bigenerina capreolus as is noted 

 under that species. 



