90 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



accelerated. There is in the megalospheric form (fig. 45), a large, 

 nearly spherical proloculum, followed by an elongated chamber of a 

 half coil in length. This second chamber is very low at its inception 

 but widens gradually toward its apertural end. The wall of the 

 proloculum and second chamber is continuous, of uniform thickness, 



in the second chamber building no 

 floor, and as already noted in Pen- 

 eroplis and 0. compressa the wall is 

 perforate. Chamber 3 is globular with 

 a single aperture exactly comparable 

 to the third chamber of 0. compressa 

 and also to the third chamber of Pen- 

 eroplis. Its shell development may 

 be seen in the figure, continuing 

 the periphery of the test from cham- 

 ber 2 and making a thin layer on 

 the apertural wall of that chamber, 

 building its floor directly on the 

 fig. 45.— early chambers of orbiculina wall of the proloculum. This same 



ADUNCA, BY TRANSMITTED LIGHT, X 50. 1 „ 1 , i , i 



development may be seen in the 

 chambers of 0. compressa (fig. 43). 



In 0. adunca chamber 4 has two apertures, thus taking on here the 

 character which was taken on in the specimen of 0. compressa in the 

 eighth chamber and in Peneroplis taken on much later than this, 

 the sixteenth chamber in Schacko's figure and in some specimens of 

 Peneroplis never reached (fig. 41). 

 0. adunca is then a much accelerated 

 species. 



Chamber 5 has also two apertures. 

 Chamber 6 has three apertures, a 

 character not attained in the speci- 

 men of 0. compressa until chamber 

 13. Chamber 7 also has three aper- 

 tures. Chamber 8 has four aper- 

 tures, as have the other chambers 

 shown in the figure. Instead of the 

 test being a compressed one as in 

 0. compressa, 0. adunca has a much 

 thicker test due to the overlap- 

 ping of the preceding chambers at 

 the sides. This is shown in the 

 section (fig. 46) at right angles to the first section. In this 

 figure P shows the proloculum and X and X' the two op- 

 posite portions of chamber 2 in cross section. At X the com- 

 mon wall between the two chambers is bent inward and at 



W 



flg. •46.— portion of vertical section of 

 Orbiculina adunca, x 50, showing early 

 stages and later involute whorls in 

 section on right bb', cc'. p, prolo- 

 culum, xx', second cornuspira-like 

 chamber cut on both sides of the pro- 

 loculum, a, a', chambers of first 

 volution. 



