Brachial Siipjiorfs hi Dielaaiiia and Zygo^pira. 77 



harranih'i and ('. iii(ir</imiJis the brachial sujiports, as worked out 

 by Davidson and Glass, consist of about iive volutions, and are 

 similar to those of Zi/gosplra, except that the transverse Ijand is 

 more posterior, since it originates near the ends of the crura. 

 This mature condition of Cwlo-'tpira, is seen to be a young condi- 

 tion of Atn/pa (Davidson), but, as the spirals are more loosely 

 coiled and the transverse band ahvays continuous, this genus 

 should l.)e regarded as valid in the evolution from Zggospira to 

 Airypa. In mature Atrypa reticularis from tlie U])per Silurian, 

 there may l)e as many as sixteen volutions in each spiral cone 

 (Davidson), but more often the number is smaller. The trans- 

 verse band in this species during its young stages is continuous, 

 l)ut 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 numl)er of 

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

 si)ecies in Yale Universit}^ Museum, having twenty-four turns of 

 the lamelhTe in each spiral. 



The ontogeny and phylogeny of the species of Zij(jospira indi- 

 cate strongly that the Atrypi(Lr had its origin in a form with a 

 centronelloid loop. A further natural conclusion from the same 

 evidence is that the Ancyloljracliia are older and more primiti\e 

 than the Helicopegmata. 



EXI'LAXATION OV PlATE X. 



Dielasma turglda. 



Figure 1. — The ceniro)ielHforiii stage o{ the looi) ; ventral view. X 6. 



2. — A later stage, showing the resorption of the anterior portion 



of the loop. X ()• 

 '.]. — Early Didasma stage, jiroduced by furtlier resofption of the 



centronelloid loop. X 6. 

 4. — Loop and crural plates of mature specimen. X (>. 

 5. — Profile of the connecting band. X <>. 

 (). — Side view of the loop, crura, and septum. X '>. J^t. T>ouis 



group, Kentuckn. 



Zi/gosj)ira rccurvirostm. 



Figure 7. — C'f)i/ro»W///'o)-H; stage of the loop. X 12. 

 S.— Profdeof saftie. X 12. 

 !•. — \ later stage ; showing partial resorption of lo<ip in front and 



greater divergence of descending ))ranc]ies. x 12. 

 h).— The same; profile. X 12. 



