4 i2 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



it overlaps the apex without touching it. At this time 

 it is plainly gyroceran, like the whole of No. 3. After 

 it touches the first whorl just beyond the apex it re- 

 mains in contact, and the inner side or dorsum of the 

 second or overlapping whorl begins to show a flatten- 

 ing as a result of this collision of the whorls. The sec- 

 tions of the orthoceran, cyrtoceran, and gyroceran 

 whorls show no such flattening in any of the speci- 

 mens examined, although hundreds of different kinds 

 have been studied. The sections are designated on 

 the plate by the same letters as the supposed lines of 

 the sections made through the tube, and although dia- 

 grammatic figures, -they give a sufficiently clear gen- 

 eral explanation of the facts observed. More specific 

 figures could have been, given in abundance and will 

 be given in the paper now in course of preparation. 1 



"Fig. 119, No. 5, shows the same phenomena as 

 No. 4. The young is at first cyrtoceran like the adult 

 whorl of No. 2, and open, then becomes gyroceran in 

 curvature and finally overlaps the apex when it has 

 arrived at the end of the first volution, but does not at 

 first touch it. Then coming into contact it acquires a 

 flattened area or faint impressed zone on the dorsum 

 or inner side of the second volution, as is shown in 

 the section No. $c. This is similar to the section of 

 No. 4 shown in No. 4*:', which represents a cut through 

 an adult whorl of the fourth class of forms. It differs 

 only in being smaller, on account of the younger stage 

 of growth at which it occurs. 



"The entire series of forms from orthoceran to nau- 

 tilian is more or less represented, even in the earliest 

 period at which the nautiloids appear, namely, in the 



ISee " Phylogeny of an Acquired Characteristic," Hyatt, Proceedings 

 American Philosophical Society, Philadelphia, XXXII., No. 143. 



