﻿431 



These terms and others of the new descriptive nomenclature, of 

 which only very few will be used in these pages, because I think it 

 will be essential to discuss them further before applying them to the 

 descriptions of cephalopodan shells, have been gradually introduced 

 in consequence of the labors of Wilder and Gage, in this country, 

 and are in a fair way of being adopted in Europe through the 

 effort of Franz Eilhard Schulze and others.* 



Terms like ventran, ventrad, ventral, dorsan, dorsad, dorsal, cen- 

 tren and centran, and so on, strike one at first as awkward and bar- 

 barous, but their utility becomes apparent, as in the case of the 

 siphuncle cited above, as soon as one begins to use them, and they 

 can be made to have an exact meaning which it is not practicable 

 to gain otherwise without the repetition in every description of the 

 same explanatory text. 



The shells of Nautiloidea and Ammonoidea are divided by trans- 

 verse partitions or septa into what are called "air chambers," and 

 the intersections or lines made by the edges of these when they 

 strike against the inner surfaces of the shell of the whorl are called 

 the sutures. Fig. 15 shows the edges of these septa as they would 

 appear in Nautilus umbilicatus (Fig. 1, p. 345) if the shell there 

 figured had been fossilized, the air chambers filled with infiltrations 

 and the outer walls of the last whorl destroyed except in the umbil- 

 icus. The outer empty chamber beyond the suture of the last 

 septum is the cast of the living chamber. The sinuous edge of 

 this is the impression left by the edge of the aperture on the right 

 side. This being a cast artificially made, is somewhat more perfect 

 than natural casts of the interiors of such forms in the rocks and 

 the spreading abutments of the septa against the inner wall are 

 broad bands. Usually, in fossils, the upper extremely thin parts of 

 these bands have disappeared, leaving only a line below correspond- 

 ing to the lower parts of the bands in this figure and more nearly 

 representing the thickness of the internal part of the calcareous 

 septum. 



* See Wilder, Science, ii, 18S1 ; Wilder and Gage, Anatomical Technology, 1SS2, and other 

 papers. Also Schulze, Biologisches Centralblatt, xiii, Nos. 1, 2, 1893; Hyatt, ibid*, Nos. 15, 

 16; and again, Schulze, Verh. d. Anat. Gcsellccli., Versani. Gottingen, 1S93, p. 101; and 

 reprint of same, Deutsche Zool. Gcsell., Gottingen, 1S93, p. 6. 



