Brachial Suppoii.s 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 

 si)ecimens illustrated in figs. 7-11. Therefore, when the loo]) 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 o.o3 mm., while the former is 

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

 vanced tluiu in fig. 9, the transverse 1jand Ijeing narrower and 

 slightly elevated posteriorly, some resorption having taken place 

 along the inner edges of the primary lamelke. 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. 

 rcciirrirostra from the lowest Trenton is retained to maturity. 

 However, in specimens of Z. reciirvirostra from the upi)er Trenton 

 the ])osteriorly curved, transverse band is not a mature feature, 

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

 in previous stages the transverse band is ventrally arched, but 

 it now l)ends 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 lamelhe, curving toward the ventral valve 

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

 lamella* 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 lamelUc 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. 



Obscrcaiions and Correlations. 



Zi/(/().sj)ir(L is the earliest S[)ire-l)caring genus known, as it is 

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

 (•onsideral)le interest, therefore, to study the development of the 

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

 1 torts in Zijf/ospira begin as a loop greatly resembling that of 

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

 ries of metamorphoses Ijefore the spirals make their appearance. 



