BRACHIAL SUPPORTS IN DIELASMA AND ZYGOSPIRA 415 



into the form represented in figure 15. 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 (figures 16 and 17) as 

 two slender converging lamellae, curving toward the ventral 

 valve and originating from the outer pointed ends of the 

 loop. These lamellae then incurve dorsally and laterally to a 

 point just posterior to the transverse band, forming the first 

 volution of a spiral (figure 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 (figure 20). The calcareous brachial supports 

 occupy about the same relative space in the shell cavity in all 

 stages of growth. 



Observations and Correlations. 



Zygospira 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 develop- 

 ment of the spirals. From the ontogeny, it is shown that 

 the brachial supports in Zygospira begin as a loop greatly 

 resembling that of Devonian Oentronella. Moreover, the 

 loop passes through a series of metamorphoses before the 

 spirals make their appearance. 



The most ancient species are Z. Nicoletti and Z. Saffordi, 

 small semi- plicate forms, in which the spirals are very rudi- 

 mentary, consisting of about one volution. In the same 

 geological horizon occurs Z. recurvirostris, having from two 

 to two and one -half turns of the lamellae in each spiral. 

 The same species from the upper Trenton has three volu- 

 tions, while in Z. modesta of the middle Lorraine there are 

 from four to five whorls (figure 25). In Z. Headi (figure 

 24) a large globose finely striated species of the upper Lor- 

 raine, there are six whorls to a cone. The geological his- 



