314 



velopnient iu that part of the organism that was most vitally affected by 

 the environment, and which must therefore indicate most perfectly the 

 lines along which the evolution proceeded. 



If the initial portion of the shell of Nautilus be examined, it will be 

 found to be characterized by a scar or cicatrix. In the same region of 

 the shells of ammonites and some Xautiloidea (Orthoceras) , instead of this 

 cicatrix, there is present a minute, bulbous or bag-like shell, attached to 

 the apex of the shell proper. If in the case of Orthoceras, as shown by 

 Hyatt (31), this bulb, or protoconch be broken away, there is exposed a 

 scar (cicatrix) precisely similar to that of Nautilus. The initial shell or 

 protoconch is therefore substantially' the same in all of the Tetrabranch- 

 iata, and is supposed to point to a "septa-less and chamberless form simi- 

 lar to the protoconch" as the common ancestor of these two great divisions 

 of the Tetrabranchiata : and possibly, as Hyatt suggests of the Cephalo- 

 poda, Pteropoda and Gastropoda (31). The protoconch represents the 

 latest of the true embryonic stages, namely the phylembryo. 



Succeeding this early stage are the stages of the shell proper. 1 In 

 Nautilus the early nepionie portion of the shell, which includes the forma- 

 tion of the first three septa, is only slightly curved ( cyrtoceraf orm ) . Up 

 to the stage of the formation of the second septum, the shell is in fact 

 nearly straight (orthorceraform). The first septum has an apically di- 

 rected caecum, and the second septum an apically directed closed tube, 

 the closed apical end of which fits into the caecum of the first septum. 

 This tube is the beginning of the siphuncle. Since the tube fits closely into 

 the caecum, the two together form a continuous tube, in which the apical 

 end or bottom of the siphuncular tube forms a partition or septum, so 

 that as Hyatt points out, the resemblance "of this early stage to the adult 

 structures of Diphragmoceras becomes perfectly clear." (31) 



In the later nepionie stages (i. e., after the formation of the third 

 septum) the shell is rather sharply bent (the gyroceran curve), so that 

 near the close of the first volution the whorl is brought back into contact 

 with the apex of the conch. This manner of growth results in leaving 

 an empty space or umbilical perforation between the two halves of the 

 first volution. In the ancient coiled Nautiloidea there appears at the be- 

 ginning of this (neanic) stage, when the whorls come into contact, a de- 



1 The stages from this point on are termed by Hyatt (31), and following him 

 by practically all paleobiologists at tbe present time, the nepionie, neanic, epliebic 

 and gerontic stages ; meaning respectively, the infantile, youthful, mature and old 

 age stages of growth. 



