DEVELOPMENT OF THE SHELL IN TORNOCERAS 437 



evidently connected with the more soluble nature of the 

 pearly shells of the nuculoids and cephalopods. 



By carefully breaking away the outer enveloping volutions 

 of a number of specimens of Tornoceras, the early parts of 

 the shell were uncovered, and found to be well preserved, and 

 therefore suitable for study. The drawings on Plate XXXIV 

 were made from the microscope, with a camera lucida. 



The protoconch (figures 1, 2, Plate XXXIV) has an axial 

 diameter of about 1.1 mm., and among several specimens 

 measured varies but little from this dimension. The verti- 

 cal diameter is a little shorter, so that the general form is 

 that of a prolate ellipsoid. The latera are prominent, and 

 exposed as central bosses in the umbilicus of a young shell. 



At what precise growth-stage the umbilicus becomes closed 

 cannot be ascertained from the material studied, but it is evi- 

 dently open during the formation of several whorls. During 

 the concrescence of the first few air-chambers, while the 

 diameter of the tube is diminishing, the tendency of the 

 umbilicus is to enlarge rapidly. Subsequent increase in the 

 tube and the greater involution of the whorls contract it, so 

 that in adult specimens it is closed, while in large and often 

 senile individuals a secondary deposit is made about the 

 umbilicus, entirely obliterating it and covering the growth 

 lines of the shell.* Evidently this formation is similar to 

 that deposited by the dorsal lobe of the mantle in Nautilus 

 pompilius. 



The axial diameter of the embryo shell is somewhat greater 

 than that of several of the succeeding air-chambers. Thus, 

 in its growth, the tube first contracts, and does not assume 

 the regular rate of increase until after the formation of at 

 least the second septum. A cross section at the first septum 

 is transversely sub-elliptical, slightly arcuate, with a longer 

 diameter two and one-third times greater than the shorter. 

 When a transverse diameter of 1.2 mm. is reached by the 



* This feature is well represented in fig. 11, pi. 127, Pal. N. Y., VII, 

 supplement. 



