70 Bcechev and kSckucIiert — TJevclopincid of the 



'r\w, ino.'-t ancient species are Z. nlcolettl and Z. saffordi, small 

 seiuiplicate forms, in which the spirals are very rudimentary, 

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

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

 turns of the lamella' in each spiral. The same species from the 

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

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

 Z. headi (fir. 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 lamella^ in each spiral. 



The transverse band connecting the primary lamella' also 

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

 tronelloid loop (fig. 7) passes into one havmg 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- 

 letti, Z. sajfordi, and in the lower Trenton varieties of Z. recurn- 

 rostra. Passing to the specimens of the latter s[)ecies, which are 

 geologically later, the band no longer joins the lamellae as far 

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

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

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

 manifestl}^ more posterior than in an_v of the older species of 

 Zjjfjospira. The transverse band is now no longer arched l)ack- 

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

 gressivel}^ more and more posterior, and the loop is gradually 

 shortened before the spirals make their appearance. The gradual 

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

 cession of the transverse band have gone on together.* 



The family Atr3^pida! includes the genera Zygospira, Glassia, 

 Odospint, Anoploiheca, Atrypa, and Day in. It is easily distin- 

 guished from all other families comprised in tiie suborder Heli- 

 copegmata, since the spirals are between the first descending 

 branches of the lamella^, while in the Spiriferidse, Nucleospirida', 

 and Athyrida^ the primary lamelkc are between the spirals. 



The gradual increase in tlie number of whorls in the spirals 

 and the })ushing l^ackward of the transverse band in the Atry- 

 pida; is carried farthest in the species of Atrypa. In Calospira 



* The extreme anterior position of the transverse band in Z. recurvi- 

 rostra is therefore of no niore than specific value, ami on tliis account 

 Anazyya Davidson cannot well be separated from Zyijospira. 



