PLATE III. 



Plattckbas (Obthontchia) attenuatum. 



Pago 6. 

 Fl». 1. DorwU view of a small sjiecimen which has Wen laterally compressed. 



Figs. S, 3. IK'rsal antl lateral views of a moi-e expanded siMjcimen, showing strong folds near the margin. 

 Fig. 4. Lateral view of the sjiecimen fig. 1, showing the folds of the margin. 



Figs. 5, 6. Lateral and dorsal views of an unusually expanded specimen, showing in'egular pustulose ridges 

 on the mirface of the exjianded iM)rtion. 



Platycebas bucculentum. 



Page 10. 

 Fig. 7. Lateral view of a 8j)ecimen which may be the young of P. bucculentum, but has the appearance of 

 an adult shell. From Canandaigua Lake, N. Y. 



PLATrCKBAS AUBICULATUM. 

 Page 7. 

 Figs. S-10. Dorsal, right and left views of a specimen, showing the form of the shell and the lateral expan- 

 sion, with its ridges. Dainen, N. Y. 



Platycerab Thetis. 



Pajte 8. 



Figs. 11, 12. Left and right lateral views of a small sjiecimen, showing the narrow form and enrolled beak. 



Figs. 13, 14. Left and right lateral views of an unusually large specimen, showing a more strongly ai-cuate 

 form. 



Figs. 15, 16. Left views of two sjiecimens of large size, showing some variation in form. From the Ham- 

 ilton group at Canandaigua Lake and Bellona, N. Y. 



Platycebas symmetbicum. 



Page 9. 

 Fig. 17. Lateral view of a large specimen, referred with some doubt to this species. From the Hamilton 



group, Canandaigua Lake, N. Y. 

 Figs. 18-21. Four views of one of the typical specimens exhibiting the general featui-es of the species. Fig. 



20 shows the symmetrically coiled beak. Fig. 21 is accidentally compressed on one side. 



From Canandaigua Lake, N. Y. 

 Figs. 22-24. Three views of a specimen, showing a loosely coiled beak (except at the apex) and strongly 



plicated margin. 

 Fig. 26. Dorsal view of a specimen, showing a strong anterior fold. From Canandaigua Lake, N. Y. 



Platycebas bucculentum. 



Page 10. 

 Fig. 2fi. Lateral view of a specimen, showing the folds of the shell on the left side. 

 Fig. 27. Dorsal view of an individual, showing the strong lateral folds. 

 Fig. 28. Dorsal \-iew of tKe specimen fig. 26. 



Fig. 29. The aperture of a large symmetrical specimen, showing a fold in the poeterior margin. Lixnngston 

 county and at Canandaigua Lake, N. Y. 



Platycebas Thetis? var. subspinosum. 



Page 9. 

 Fig. 80. View of a large flattened specimen, possessing many of the features of P. Thetis, but moi-e robust 

 and with the ajierture contracted on the right side and strongly plicated. There are a few short 

 spines at the summit of the volution. From the Hamilton gi-oup at Canandaigua Lake, N. Y. 



