154 CALIFORNIA ACADEMY OF SCIENCES. [Proc. ^d Ser. 



EXPLANATION OF PLATE XIL 



The Development of B acuities chicoensis Trask.' 



Upper cretaceous, Chico beds, Jordan ranch, Arroyo del 

 Vall6, eight miles southeast of Livermore, Alameda County, 

 California. 



Fig. 226. Protoconch, front view, diameter 0.48 mm. ; enlarged 



15 times. 

 First septum, showing siphonal caecum. 

 Second septum, at diameter 0.58 mm. 

 Larval shell, at one-fourth of a revolution, diameter 



0.58 mm.; 15 times enlarged. 

 Sixth septum, at one-half revolution. 

 Larval shell, showing the embryonic constriction, and 



the first larval body-chamber; 15 times enlarged. 

 Larval shell at three-quarters of a revolution, diameter 



0.83 mm.; enlarged 15 times. 

 Larval shell, showing the ornamentation of the embry- 

 onic and early larval stage, and the ananepionic 



body-chamber; enlarged 15 times. 

 Shell at end of the second larval stage, diameter 1.6 



mm.; 15 times enlarged. 

 Larval shell, showing the periodic swelling of the 



siphuncle. Diameter i.oomm.; enlarged 15 times. 

 Figs. 239-240. Early adolescent stage, showing the unsynimetric 



shape of the larval coil, and the contraction of the 



shell at the beginning of this stage; enlarged 15 



times. 

 Fig. 241. Composite drawing from several specimens, showing 



the development of the septa from the embryonic 



into the adolescent stage; enlarged 5 times. 



' These drawings are copied from a paper by J. P. Smith, "The Larval Coil of 

 Baculites," American Naturalist , Vol. XXXV, p. 39, Jan., 1901. The numbers on this 

 plate do not correspond to the originals of Smith's plates, since not all his figures are 

 reproduced here. 



