246 SYNOPTIC COLLECTION. 



The markings of the protoconch are carried over contin- 

 uously to the beginning of the nepionic stage, proving 

 conclusively that the shell or conch was formed on the 

 edges of the protoconch. Usually the protoconch is 

 broken off and in these cases a scar is left. Fig. 7 (O. 

 unguis) shows the scar and the conch of the apex. Fig. 

 8 is a view of the apex of the same species after the proto- 

 conch has been accidentally broken off, fracturing the 

 outer shell and exposing the scar. 



The young has a large siphuncle which afterwards in 

 the process of growth becomes contracted, but never 

 develops the cone-in-cone structure, or an endosiphuncle. 

 This tends to prove that Orthoceras descended from 

 forms with a large siphuncle and that the small siphuncle 

 is a secondary condition. 



The position of the siphuncle is usually near the center 

 of the septa, as seen in No. 587, which represents a portion 

 of the adult shell. The septa are plain and concave 

 towards the living chamber. The shell is straight and 

 unornamented, with a simple aperture. It is marked by 

 that genuine simplicity which characterizes primitive 

 forms. 



The tendency of the primitive straight shell to become 

 coiled is shown in Cyrtoceras and Gyroceras. Cyrtoceras 

 (PI. 588) is more or less curved, but is never coiled. The 

 septa are concave towards the forward part, the aperture 

 is large, and the ventral sinus is distinct. In Gyroceras 

 (PI. 589) the process of coiling begins, but the whorls are 

 not in contact, so that the result is a loose spiral. The 

 aperture is simple and the septa are plain and concave. 



There are fossil forms among the Nautiloids in which 

 the whorls are closely coiled, as in Nautilus dekayi Mor- 

 ten (No. 590, showing septa and siphuncle) and Nautilus 

 intermedius Sow. (No. 591). In some species these whorls 

 are visible (No. 591 ; No. 592, Nautilus umbilicatus Lis- 

 ter; section of shell), and in consequence of this mode of 

 growth the species has a deep umbilicus on either side. 



