284 SYNOPTIC COLLECTION. 



ports, but has a band or circlet of tentacles. This is the 

 larval stage corresponding to the adult primitive Gwynia, 

 and is therefore called the Gwyniform stage (PI. 699, Ai). 

 Next the median septum appears, which is the Cistelliform 

 stage (Bi). The only advance over this condition shown 

 by the adult Cistella (PL 699, B) is the calcification of 

 the band which bears the tentacles, and its attachment to 

 the crura. 1 The third stage shows a ring on the septum 

 (Ci, Cai, side view of same) which is the Bouchardiform 

 stage. A still more advanced condition is seen in the 

 Megerliniform stage (Di, Dai, side view) which has 

 the septum ring and in addition to these the " prongs " 

 of the descending branches. It has been shown ' 2 that 

 in " all genera where the median septum is highly devel- 

 oped the calcification of the lamellae of the brachidium 

 begins quite as soon from the lateral wall of the septum 

 as from the crural bases on the hinge-plate. Calcification 

 thus proceeds both posteriorly and anteriorly." 



The Magasiform stage exhibits the completion of the 

 descending branches (Ei). This stage and also the Mag- 

 aselliform stage (Fi) which follows, show the union of the 

 ascending branches, and in this particular differ from the 

 adult Magas (E) and Magasella (F). 



The Terebratelliform stage (Gi) is similar to the adult 

 of Terebratella (G). At last the final stage is reached, 

 as we have already stated, by the resorption of the con- 

 necting band and the septum (Hi, Magellania venosa 

 Sol.). 



The family of Atrypidae is represented by Zygospira 

 and Atrypa. Zygospira is the most primitive spire- 

 bearing genus known. Figs, i-n of PI. 700 represent 

 Zygospira recurvirostra, Z. modes ta, Z. headi, and No. 

 701 is a small colony of Zygospira modesta Hall, in 

 which the shells are preserved in natural position. 



1 Beecher, Trans. Conn. Acad. Sci., IX, part 2, 1895, p. 393. 



2 Hall and Clarke, Nat. Hist. N. Y. Pal., VIII, part 2, 1894, p. 

 305- 



