414 STUDIES IN EVOLUTION 



ventrally concave, primary lamellae, rapidly increasing in 

 width from the thin crural plates to near the centre of the 

 valve, where they unite, forming a plate with a central 

 angular ridge. The anterior end of the plate is pointed, as in 

 Centronella. 



In a specimen about 2 mm. in length (figures 9, 10), the 

 primary lamellae are practically of the same form as in the 

 preceding, but much of the original central portion of the loop 

 has been resorbed, so that the lamellae are connected by a 

 short but comparatively wide, ventrally arched, transverse 

 band. The lamellae, or descending branches, are also more 

 spreading anteriorly, and there is a slight deflection at the 

 crural points which becomes more and more pronounced as 

 growth progresses. 



In the next stage (figure 11), which has a length of 2.33 

 mm., the descending branches are more diverging, and the 

 transverse band is longer and more broadly excavated in 

 front. 



The succeeding stages here described are based upon mate- 

 rial derived from near the top of the Trenton, where the 

 specimens of this species are usually larger and more trans- 

 verse than those from near the base of the Trenton, which 

 is the horizon of the specimens illustrated in figures 7-11. 

 Therefore, when the loop in figure 9 is compared with that 

 of figure 12, it is seen that the latter is much the wider, from 

 the greater size and breadth 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 figure 12 is somewhat more advanced 

 than in figure 9, the transverse band being narrower and 

 slightly elevated posteriorly, some resorption having taken 

 place along the inner edges of the primary lamellae. Further 

 resorption in same direction produces the brachial support 

 illustrated in figure 14. This form of loop in Z. Nicoletti, 

 Z. Saffordi, and Z. recurvirostris from the lowest Trenton is 

 retained to maturity. However, in specimens of Z. recurvi- 

 rostris from the upper Trenton the posteriorly curved, trans- 

 verse band is not a mature feature, since it becomes changed 



