Brachial Xupjuni* in Dielasma and Zygospira. 75 



derived from near the top. of the Trenton, where the specimens 

 of this species are usually larger and more transverse than those 

 from near the base of the Trenton, which is the horizon of the 

 specimens illustrated in figs. 7-11. Therefore, when the loop in 

 fig. 9 is compared with that of fig. 12, it is seen that the latter 

 is much the wider, from the greater size and breath of the shell, 

 which has at this stage a length of 3.33 mm., while the 'former is 

 but 2 mm. long. The loop in fig. 12 is somewhat more ad- 

 vanced than in fig. 9, the transverse band being narrower and 

 slightly elevated posteriorly, some resorption having taken place 

 along the inner edges of the primary Iamella3. Further resorp- 

 tion in same direction produces the brachial support illustrated 

 in fig. 14.. This form of loop in Z. nicoletti, Z. saffordi, and Z. 

 recurvirostra from the . lowest Trenton is retained to maturity. 

 However, in specimens of Z. recurvirostra from the upper Trenton 

 the posteriorly curved, transverse band is not a mature feature, 

 since it becomes changed into the form represented in fig. lo- 

 in previous stages the transverse band is ventrally arched, but 

 it now bends dorsally, and remains so during subsequent growth 

 until near maturity, when the sinus of the dorsal valve causes it 

 to assume a sigmoid curve. 



The spirals next begin to develop (figs. 16 and 17) as two 

 slender converging lamellae, curving toward the ventral valve 

 and originating from the outer pointed ends of the loop. These 

 lamellti 4 then incurve dorsally and laterally to a point just pos- 

 terior to the transverse band, forming the first volution of a 

 spiral (fig. 18). . In this manner further growth and elongation 

 of the lamellae continue until maturity is attained, when there 

 are about three volutions in each spiral cone (fig. 20). The cal- 

 careous brachial supports occupy about the same relative space 

 in the shell cavity in all stages of growth. 



Observations and Correlations. 



Zi/(/ox{>ira is the earliest spire-bearing genus known, as it is 

 found in the Birdseye limestone of the Trenton period. It is of 

 considerable interest, therefore, to study the development of the 

 spirals. From the ontogeny, it is shown that the brachial sup- 

 ports in Zygospira begin as a loop greatly resembling that of 

 Devonian Centronella. Moreover, the loop passes through a se- 

 ries of metamorphoses before the spirals make their appearance. 



