76 Beecher and Schachert Development of the 



The most ancient species arc /. ni.cnlctti uncl Z. saff'ordi, .small 

 semiplicate forms, in which the spirals arc very rudimentary, 

 consisting of about one volution. In the same geological hori- 

 zon occurs Z. recurvirostra, having from two to two and one-half 

 turns of the lamellae in each spiral. The same species from the 

 upper Trenton has three volutions, while in Z. modesto of the 

 middle Lorraine there ara from four to five whorls (fig. '25). In 

 Z. headi (fi:. 24), a large globose finely striated species of the 

 upper Lorraine, there are six whorls to a cone. The geological 

 history, therefore, shows a gradual increase of from one to six 

 turns of the lamellae in each spiral. 



The transverse band connecting the primary lamelltr also 

 undergoes a series of changes. It has been shown that the cen- 

 tronelloid loop (fig. 7) passes into one having the lamella? joined 

 by a posteriorly directed, transverse band (fig. 14). This form 

 of loop is retained as a mature feature in the brachia of Z. nico- 

 lettij Z. saffordi, and in the lower Trenton varieties of Z. recur ri- 

 rostra. Passing to the specimens of the latter species, which are 

 geologically later, the band no longer joins the lamella? as far 

 anteriorly as in the older variety (fig. 20). The point of connec- 

 tion in Z. modesta is variable (figs. 25 and 26), but is usually 

 more posterior than in Z. recurvirostra, while in Z. headi it is 

 manifestly more posterior than in any of the older species of 

 Zi/</ox}>ir<i. The transverse band is now no longer arched back- 

 ward, but is just the reverse (fig. 24), while its position is pro- 

 gressively more and more posterior, and the loop is gradually 

 shortened before the spirals make their appearance. The grad ual 

 increase in the number of the whorls in each spiral and the re- 

 cession of the transverse band have gone on together.* 



The family Atrypidse includes the genera Zygospira, Gln^'m, 

 Codospira, Anoplolheca, Atryp, and Duyia. It is easily distin- 

 guished from all other families comprised in the suborder Heli- 

 copegmata, since the spirals are between the first descending 

 branches of the lamella?, while in the Spiriferidie, Nucleospirida?, 

 and Atbyrida? the primary lamella 1 are between the spirals. 



The gradual increase in the number of whorls in the spirals 

 and the pushing backward of the transverse band in the Atry- 

 pida; is carried farthest in the species of Atrypa. In G/7a*/>/m 



*The extreme anterior position of the transverse banding, recurvi- 

 rostra is therefore of no more than specific value, and on this account 

 Anazyt/d Davidson cannot well be separated from Zt 



