BRACHIAL SUPPORTS IN DIELASMA AND ZYGOSPIRA 413 



descending branches diverge and the loop is two-fifths the 

 length of the dorsal valve. 



The natural inferences to be drawn from the development 

 of the loop in Dielasma are, that Oentronella represents a 

 larval or immature condition of the higher genera, and that 

 the centronelloid loop is the primitive type in the Terebra- 

 tulidse. Therefore, as Centronella and the closely related 

 genus Rensselceria are the only early punctate terebratuloids 

 known, and as they have the primitive type of loop, there 

 arises the question of the validity of the Upper and Lower 

 Silurian species with recurved loops, referred to Waldheimia 

 and Hallina. 



Hall and Clarke (Pal. N. F., vol. viii, part ii, pp. 147- 

 153, not yet published) describe and figure the brachial sup- 

 ports in Hallina, showing that both H. Nicoletti Winchell 

 and Schuchert and H. Saffordi Winchell and Schuchert are 

 provided with short spires of about one volution, connected 

 by a transverse band, as in Zygospira. In removing the ven- 

 tral valve and exposing the loop from that side, as is often 

 done, the short spiral lamellae 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 re- 

 spects agreed with the diagnosis of Dayia. These facts 

 indicate that the specimens described by Davidson as Wald- 

 heimia Maivii (Fossil Brachiopoda, Supp. vol. iv, pt. v, pi. 

 iv, figs. 1-3) are the young of Dayia navicula Sowerby, sp. 

 (ibid., pi. v, figs. 1-4). 



Development of the Brachial Supports in Zygospira 

 recurvirostris. 



The smallest specimen in which the internal structure was 

 observed measures about 1.33 mm. in length (Plate XXVI, 

 figures 7, 8). The brachial supports consist of two straight, 



