82 SchucJierf — Sitell of Z>/go.y)ira Bccnrvirosfra. 



(lissiinilar exteriors are seen to have resulted from the same 

 stock. Zt/gospira attained the greatest development in the Lor- 

 raine grou]) of the Ohio valley. The species are usually very 

 abundant in individuals, often forming groups several inches in 

 diameter. 



Ex IT, A NATION OF PlATE XI. 



Figure 1. — Dor,sal view of the smallest sj^ecimen observed of Zygospira 

 recurvirostra Hall. The fold and sinus are just visible alony 

 the anterior margin of the shell. X 12. 



2. — A later stage, where the deltidial plates have begun to develo[). 

 X 12. 



o. — A specimen in which the i)lications are developed unusualh' 

 early. X 12. 



4. — A si)ecimen of larger growth than figure .3, in which the plica- 

 tions are slow in developing. X 12. 



0-8. — Different individuals to show the progression of growth in 

 the deltidial plates and the e.xtent of the plications over 

 the umbones. X 12. 



!). — A mature example. The pedicle foramen is posterior to the 

 deltidial plates, encroaching on the umbone of the ventral 

 valve. X (>. 



ln-9n. — A series of profiles of the specimens, figures 1-9, re- 

 spectively, showing the change in the convexity of the 

 valves and the incurvature of the ventral beak. 

 10. — A specimen preserving the lines of growth very clearly, of 

 which the stronger ones only are figured. The apex of 

 the ventral beak has been worn away by abrasion and 

 resorption by the pedicle, a, protegulum ; h, the first 

 articulate stage; c, the stage in which the fold and sinus 

 are developed; (7, the plicated stage; f, deltidial i)lates; 

 /, pedicle opening. X 25. 



11, 1 iff. — An unusually large specimen of [HuUiaa ^ Zygospini 



nlcoletli W. and S., in which a few plications are develoi)ed 

 on each side of the fold and sinus. X 6. 



12, 12a. — Dorsal and profile views of one of the types of {Halllnn =) 



Zygospira nicolctti AV. and S., the form most commonly 

 observed. X (>. 

 • KJ, V.'xt. — Dorsal and profile views of the type of {HalUna ==) Zggo- 

 splrti mffordl W. and S. Tlii.s sjiecies is closely related to 

 Z. recur riroslra. X •>. 



