C. E. Beecher — Development of Shell in Tomoceras. 73 



At what precise growth stage the umbilicus becomes closed 

 cannot be ascertained from the material studied, but it is 

 evidently open during the formation of several whorls. Dur- 

 ing 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 um- 

 bilicus, entirely obliterating it and covering the growth lines 

 of the shell* Evidently this formation is similar to that de- 

 posited by the dorsal lobe of the mantle in N autilus jpompilius. 



The axial diameter of the embryo shell is somewhat greater 

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

 the tube, in its growth, 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 trans- 

 versely subelliptical, slightly arcuate, with a longer diameter 

 two and one-third times greater than the shorter.. When a 

 transverse diameter of l-2 mm is reached by the larval shell, the 

 outline of a section is lunate, but the proportions of length 

 and height are not materially changed. A section of the adja- 

 cent whorl is still more arcuate, as shown in figure 6, and in 

 an adolescent specimen 11 •5 mm in diameter it is seen that the 

 diametral relations have become interchanged, and that the 

 outer whorl is elliptical in a vertical direction, and excavated 

 by the inner whorl to nearly half its longest diameter, making 

 the shell in all nealogic and ephebolic stages decidedly com- 

 pressed in outline (figure 13). 



The first septum (figures 1, 2, 4) is moderately concave, and 

 extends nearly to the axis. The suture is simple, being nearly 

 in a single plane, without apparent lobes or saddles. Occa- 

 sionally, the internal mould shows a siphonal lobe due to the 

 breaking away of the extremely thin filling between the siphon 

 and ventrum, but perfect specimens determine this to be an 

 accidental condition. 



In the section represented in figure 12, the first two septa 

 are much thicker than those immediately succeeding, a feature 

 also noticeable on the exterior of the internal mould. Like- 

 wise, the first and second air chambers are deeper than the 

 three or four following. With these exceptions, the septa and 

 air chambers are generally uniform in their progression until 

 the adult stage. 



It has already been noted that the first septum is extremely 

 simple, without apparent lobes or saddles. In the second sep- 



* This feature is well represented in figure 11, Plate 127 of Pal. N. T., vol. vii, 

 Supplement. 



