METAZOA VERMES. 285 



The brachial support in the young Zygospira is a Cen- 

 tronelliform loop (PI. 700, fig. i ; fig. 2, side view of the 

 same). With the growth of the shell the descending 

 branches of the loop diverge, while the resorption of 

 the central portion of the loop causes the formation of 

 the cross band or jugum, as seen in figs. 3, 4. Both 

 the loop and the jugum become more slender (Fig. 5), 

 while the projections on the descending branches of the 

 loop are the beginnings of the spiral cones. These pro- 

 jections grow longer (fig. 6) and begin to coil. Fig. 7 

 represents a young individual in which there are one 

 and a half turns to each spiral. In the mature form 

 (fig. 8) there are about three volutions in each spiral. 

 The adult Zygospira modesta Hall (figs. 9, 10) has five 

 volutions, while the species Z. headi Billings (fig. n), 

 has six whorls. 



At the same time that the whorls have increased in 

 number, the jugum has moved posteriorly (figs. 9-11), 

 until in Z. headi it is seen to be posterior to the brachia 

 (fig. n). The spiral arms and the jugum are both seen 

 in the two microscopic preparations (No. 702) of the 

 shell of Zygospira. 



The largest number of whorls of any member of the 

 family Atrypidae is reached in the genus Atrypa. In 

 A. reticularis Linn., from the Silurian, sixteen volutions 

 have been counted in each cone, while in Devonian 

 specimens (Nos. 703, 704 ; PI. 705) twenty-four volu- 

 tions have been found. These are directed towards the 

 median dorsal region and fill the brachial cavity. The 

 cross band or jugum is continuous in the young but 

 becomes disunited in mature specimens. No. 704 shows 

 the club-shaped ends of the jugum posterior to the apices 

 of the whorls (see PI. 705). 



One of the most specialized families of the Telotremata 

 is the Spiriferidae, represented by Spirifer (No. 706, S. 

 grannlosus Conrad; No. 707, S. mucronatus Conrad). 

 Here we have a broad shell with well developed cardinal 



