1892.] NATURAL SCIENCES OF PHILADELPHIA. 137 



the front view is shown in fig. 3. In this latter the form of the 

 septum may be readily seen, and also the position of the siphuncle. 

 It will be noticed that the perforation of the siphuncle is large (0*07 

 ram. in diameter), this diameter of the perforation remains quite 

 constant throughout the spiral portion of the shell. The form of 

 the first septum is lunate, 0'55 mm. broad by 0*15 mm. high 

 and the lateral extremities reach to near the axis of the spiral. 

 The next following septa have successively smaller lateral 

 diameters and larger vertical diameters, so that the successive 

 whorls become rapidly less and less enveloping. The form of 

 the septa at the same time is gradually changing; the lunate form 

 of the first septum gives place to a broadly elliptical form concave 

 on the inner side ; this in turn passes into a more and more circular 

 form, until it becomes completely circular in the straight portion of 

 the shell. Thus Plate IX, fig. 8, shows the fourth septum to be 0-52 

 ram. broad by 0-21 mm. high, the total vertical diameter of the shell 

 at this point being 0'56 mm. or about the same as the lateral. The 

 seventh septum measures 0*47 mm. by 0'23 mm. high ; the thirteenth, 

 Plate IX, fig. 7, measures 0-45 mm. broad by 0*28 mm. high. 

 Finally the seventeenth septum, Plate IX, fig. 6, measures 0"50 

 mm. broad by 0'40 mm. high, showing that the minimum breadth has 

 been passed, and in fact at about the fourteenth septum the breadth 

 seems to be least. It will be noticed that the surface of contact 

 between the inner and outer whorls rapidly diminishes from 0'55 

 mm. at the first septum to 0"2() mm. at the seventeenth septum, and 

 thence rapidly to the straight portion which begins somewhere 

 between the twentieth and twenty-fifth septa. This surface of contact 

 may be readily traced on the inner whorls of the shell and these 

 traces are indicated in Plate IX, figs. 6, 7 and 8. In the straight 

 portion of the shell the form of cross section passes gradually from 

 a circular to an ovoidal, laterally compressed form and finally in 

 the adult into a somewhat triangular form, acute ventrally (the 

 side on which the siphuncle is located) and flattened dorsally. The 

 cross section of the shell is thus seen to be first lunate, laterally 

 elongated ; then successively laterally elliptical, circular, laterally 

 compressed, and finally somewhat triangular. These changes 

 up to the circular form take place very rapidly ; the succeeding 

 changes from the circular form to the triangular form are very 

 gradual. In this respect the shell shows very rapid development in 

 the spiral stage and gradual development on quite new lines in the 

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