74 BeecJier and Schacliert — Development of the 



idity of the Upper and Lower Silurian species with recurved 

 loops, referred to Walilheimia and HalUaa. 



Hall and Clarke (Pal. N. Y., vol. viii, part ii, pp. 147-153, not 

 yet published) describe and figure the brachial supports in 

 Hallina. showing that both H. nicolettl, Winchell and Schuchert 

 and H. saffordi, \V. and S. are ])rovided with short spires of 

 about one volution, connected by a transverse band, as in Zi/go- 

 Kpira. h\ removing the ventral valve and exposing tlie loop 

 from that side, as is often done, the short spiral lamellte have 

 been overlooked. Similar observations have been made by the 

 present writers, so that the systematic position of these forms is 

 now established. 



Specimens of WaldJieimia bicarhuda Angelin, from the Upper 

 Silurian of Gotland, were also examined. I'hey were found to 

 possess well-defined s})iral cones, and in other respects agreed 

 with the diagnosis of Day la. These facts indicate that the 

 specimens described b}^ Davidson as Waldheimia maioii (Fossil 

 Brachiopoda, Sup]), vol. iv, part v, pi. iv, figs. 1-3) are the 

 young of Dayla narlnda Sowerl)y, sp. (ibid., pi. v, figs. 1-4). 



Development of the bnifliial Supports la Zygospira recti.rviro.dra. 



The smallest specimen in which the internal structure was 

 observed measures ul)out 1.33 mm. in length (plate x. figs. 7, <S). 

 The l)rachial su[)ports consist of two straight, ventrall}' concave, 

 primary lamelke, rapidly increasing in width from the thin 

 crural plates to near the center 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 ram. in length (figs. 9, 10), the primary 

 lamelke are practically of the same form as in the j)receding, 

 but much of the original central portion of the loop has been 

 resorbecl, so that the lamella? are connected by a short but com- 

 paratively wide, ventrally arched, transverse band. The lamella', 

 or descending liranches, are also more spreading anteriorly, and 

 tluTc is a sliglit deflection at the crural points, which hecomcs 

 more and more pronounced as growth i)rogresses. 



In the next stage (fig. 11), which has a length of 2.33 mm., the 

 descending branches are more diverging, and the transverse ))and 

 is longer and more broadly excavated in front. 



The succeeding stages here described are leased upon material 



