140 PALEONTOLOGY 



more or less included by the outer. By tighter and 

 tighter coiling (a continuation of the same process which 

 led from cyrtocone to ophiocone) the umbilicus becomes 

 smaller and smaller until we reach the sphcerocone with no 

 umbilicus or a very small one, as in Nautilus. 



If these stages represent a real evolutionary series, we 

 ought to find (i) that they follow one another in time, 

 and (2) that the ontogeny of forms in the later stages 

 should show traces of the earlier stages. 



As to the first point, remembering the imperfection of 

 the palaeontological record, and the fact that different 

 stocks must have gone through similar developments at 

 different times, we must not expect too much. Orthocones 

 are known from the Lower Cambrian to the Trias, and 

 are pretty equally distributed though getting scarcer after 

 the Carboniferous. Cyrtocones are known from the upper- 

 most Cambrian, but are most abundant in the Silurian and 

 Devonian ; gyrocones, rare in the Silurian, are abundant 

 in the Devonian, and then become rarer ; ophiocones, 

 though known from the Ordovician and Silurian, are rare 

 until the Devonian and Carboniferous; while sphaerocones 

 first appear in the Carboniferous and continue to the 

 present. Thus, on the whole, the known sequence in 

 time agrees with the expected sequence. 



As to the second point, allowing for the tendency to 

 hurry through ancestral stages during ontogeny, the 

 evidence is satisfactory. The theoretically earlier stages 

 do often precede the later in ontogeny, as may be seen in 

 several illustrations (Figs. 41, b\ 45, a}. 



At the same time, ontogeny shows evidence of 



