71 Beechcr and Schachert Development of the 



idity of the Upper and Lower Silurian species with recurved 

 loops, referred to Waldheimia and ILiU'nm. 



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

 yet published) describe and figure the brachial supports in 

 Jhidina, showing- that both H. nicoletti, Winchell and Schuchert 

 and H. xnffordi, W. and S. are provided with short spires of 

 about one volution, connected by a transverse band, as in Zyyo- 

 Kjiira. In removing the ventral valve and exposing the loop 

 from that side, as is often done, the short spiral lamella} 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 Waldheimia bicarinata Angelin, from the Upper 

 Silurian of Gotland, were also examined. They were found to 

 possess well-defined spiral cones, and in other respects agreed 

 with the diagnosis of Dayi'a. These facts indicate that the 

 specimens described by Davidson as Waldheimia maw it (Fossil 

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

 young of Dayia navieida Sowerby, sp. (i&*VZ.,.pl. v, figs. 1-4). 



Development of the brachial Supports in Zygospira recurvirostra. 



The smallest specimen in which the internal structure was 

 observed measures about 1.33 mm. in length (plate x, figs. 7, 8). 

 The brachial supports consist of two straight, ventrally concave, 

 primary lamella}, 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 mm. in length (figs. 9, 10), the primary 

 lamella are practically of the same form as in the preceding, 

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

 ivsorbed, so that the lamella} are connected by a short but com- 

 I >M ratively wide, ventrally arched, transverse band. The lamella 4 , 

 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 ( fig. 1 1), which has a length of 2.3-'} mm., the 

 descending branches are more diverging, and the transverse hand 

 is longer and more broadly excavated in front. 



The succeeding stages here described are based upon material 



