204 



THE AMEBIC AN NATURALIST [Vol. XLVII 



where the young plates are added. In Ammonites, as 

 Placenticeras, in which the sutures of the septa are com- 

 plex, often in a very high degree, we find that at the inner, 

 or umbilical, portion of each individual septum a simpler 

 condition exists, and greater complexity is attained in 

 passing from the ventral portion of the septum outward, 

 or dorsally. This simpler ventral portion in an adult 

 can be compared with the simpler condition in a whole 

 septum of the young, or with the septum of the adult in 

 a more primitive and geologically older representative 

 of the group. 



Parallelism is a most important principle and was 

 constantly used by Professor Hyatt in his studies. Par- 

 allelism is the taking on of a similar form in independent 

 lines of descent. It may help one in explaining the 

 origin of structures, but is sometimes confusing as indi- 

 cating a basis of relationship which is misleading. In 

 Crustacea the recent isopod Serolis closely resembles a 

 trilobite. The uncoiled gastropod shell Vermetus closely 

 resembles the worm Serpula. Spondylus, Chama and 

 Mulleria amongst Pelecypoda, and Davidsonella and 

 Derbya among Brachiopoda are all attached by the cal- 

 careous fixation of one valve and closely resemble 

 Ostrea, which has a similar habit of life. The complex 

 septa of the Tertiary nautiloid Aturia closely resemble 

 those of the Devonian ammonoid Goniatites. Echini 

 with imbricating coronal plates were considered as 

 related on account of this character, but this structure 

 appears in several independent lines in the group. The 

 recent deep-sea Echinothuriidai have many rows of 

 ambulacral plates only in the peristome. By this char- 

 acter they have been associated with the Palaeozoic 

 Lepidoeentrida? which have the same feature. I believe 

 however that it is purely a parallelism and not a basis 

 for genetic connection. 



Larval adaptation is the term applied to special fea- 

 tures built up as youthful adaptations and which are not, 

 therefore, of phylogenetic significance. Such adapta- 

 tions are a marked feature of certain groups as the ven- 

 tral spurs developed in the embryonic glochidial stage of 



