Br^ACHIAL SUPPORTS IN DIELASMA AND ZYGOSPIllA 417 



the transverse band is more posterior, since it originates near 

 tlie ends of ttie crura. This mature condition of C'oAospira 

 is seen to be a young condition of Atrypa (Davidson), but, 

 as the spirals are more loosely coiled and the transverse band 

 always continuous, this genus should be regarded as valid in 

 the evolution from Zygospira to Atrypa. In mature Atrypa 

 reticularis from the Upper Silurian, there may be as many as 

 sixteen volutions in each spiral cone (Davidson), but more 

 often the number is smaller. The transverse band in this 

 species during its young stages is continuous, but in the 

 adult condition it seems to be usually disunited in the 

 middle. This feature becomes a distinct adult character in 

 the Devonian specimens, which also have a greater number 

 of whorls in the spirals, as shown in a Chemung specimen of 

 this species in Yale University Museum, having twenty-four 

 turns of the lamellae in each spiral. 



The ontogeny and phylogeny of the species of Zygospira 

 indicate strongly that the Atrypidoe had its origin in a form 

 with a centronelloid loop. A further natural conclusion 

 from the same evidence is that the Ancylobrachia are older 

 and more primitive than the Helicopegmata. 



27 



