MUSEUM OF COMPARATIVE ZOOLOGY. 85 



in, and the inferior lateral cells appear. * The whorl continues to en- 

 large, and finally these also retire to make way for the first auxiliary 

 lobes. This occurs in Deroceras planicosta upon the last quarter of the 

 second volution ; but in Arnioceras semicostatum the development is 

 much accelerated, the first auxiliary lobe appearing on the first cpjarter of 

 the second volution. The first auxiliary cell succeeds these lobes in the 

 latter species, certainly as early as the second quarter of the third whorl, 

 and in the former they are delayed until the second quarter of the 

 fourth. 



The peculiar minor lobe dividing the superior lateral cells, the first 

 decisively Ammonitic characteristic assumed by the sutures, is fully ex- 

 pressed in Arnioceras semicostatum, on the latter part of the third vo- 

 lution, while in Deroceras planicosta the minor lobes are hardly 

 apparent on the latter part of the fourth. 



Further than this it is not my present purpose to carry the history of 

 the development of the sutures. From this point, or rather after the 

 first two whorls, the sutures should be studied in connection with special 

 series and groups of series. 



D'Orbigny, who studied more thoroughly than any other author the 

 mode of apparition of the auxiliary lobes upon the umbilical side, decid- 

 ed that they were due to the increase in breadth of the whorl, and the 

 facts here recorded confirm the accuracy of his statements. 



My observations do not lead me to recognize the so-called Ceratitic 

 period of other writers. If there is anything peculiar to the Ceratites, 

 it consists in the presence of an indefinite number of small, club-shaped, 

 serrated lobes. No corresponding features exist in the young Ammonite, 

 not even in Amm. floridus, as figured by Hauer. The smooth and deeply 

 sinuous sutures of the young Ammonite, such as Arnioceras semicosta- 

 tum, have a faint resemblance to those of the adult of Goniatites Hyas, 

 from Rockford, Indiana, and to some adult Clydonites or young Cera- 

 tites. These are mere similarities, however, which do not impress the 

 observer as anything more than mimetic changes, such as are observable 

 everywhere between the structures and forms of distinct, but genetically 

 connected series. The fundamental characteristics of the simpler and 

 earlier-occurring Xautiloids and Goniatites are very distinctly repeated 

 in the young Ammonite, but beyond this comparisons can only be safely 

 made by tracing series downward. I have succeeded, by the aid of the 



* Piute I, Fig. 8. Plate II, Fig. 9. 



