392 G. II. Girty — Fauna found in the Devonian 



specimens seem to show a broad open umbilicus, a single indi- 

 vidual contains evidence of this opening having been closed by 

 a deposit from the inner lip. Volutions regularly rounded, 

 with a nearly circular transverse section. 



Surface with revolving lines or ridges, ornamented only by 

 striae of growth which are regularly arranged. Band broad 

 (nearly l-J- mm ), slightly elongated, concave, peripheral, and only 

 about 6 mm or less in length, distinctly bounded above, below, 

 and behind by a raised line. The striae above the band bend 

 backward to meet it, and likewise those beneath, very strongly 

 so. The striae upon the band also are rather strongly Hexed 

 backward. Just behind the rounded end of the band the striae 

 also seem to cease and the shell appears to be perfectly smooth. 

 but the striae, without however the band, sometimes reappear 

 again upon the earlier volutions, especially it seems where pro- 

 tected in some way by the shape of the shell. This loss of 

 surface ornamentation just before entering upon the banded 

 stage is probably due to erosion, but at the same time the con- 

 dition appears to be a persistent one. Diameter 7 mm , height 



7mra 



Locality and position. At the base of the Devonian black 

 shale, supposed to be of the Genesee age, near Jefferson ville, 

 Montgomery Co., Kentucky. 



The specimens, of which quite a number have been collected, 

 show great uniformity in size, and other characters lead to the 

 conclusion that the type is a constant and normal one. The 

 small size in conjunction with the short duration of the banded 

 period, however, suggest that the collection may represent a 

 sort of colony of immature individuals. 



I do not know of any species of the same geologic age with 

 which comparison may be made, but it has occurred to me 

 that we may have here young examples of Pleurotomaria plena* 

 or P. arata-f of the Upper Helderberg group or of P. rugulata% 

 of the Goniatite limestone. Plethospira socialis certainly has 

 not the ornamentation of P. arata and has a lower spire. 

 Similarly it seems to be a smaller species than even young 

 three-whorled examples of P. plena, and with a lower spire, 

 though otherwise strongly suggesting Hall's species. P. rugu- 

 lata in size and other characters seems more closely related 

 than perhaps even P. plena, but even it is larger and propor- 

 tionately higher. 



Although originally described from the Marcellus shales, P. 

 rugulata has also been identified by Clark from rocks of the 

 Genesee period (Bull. U. S. Geol. Surv., No. 16, pp. 23, 33). 



*Geol. Surv. New York, vol. v, pt. 2, 1879, p. 66, pi. 17, figs. 11, 12, 13. 

 f Ibidem, p. 64, pi. 17, figs. 1-8. 

 % Ibidem, p. 75, pi. 20, figs. 1-7. 



