240 DKriYOSlH)NGID/E. 



PLATE XXI. 



PKISMODICTYA, gen. nov. 



Page 79. 

 (See Plates XVII, XVIII, XIX, XX, XXVII, XXXIII, XXXIV, XXXV, XXXVl, XLI, XLII, LV.) 



Prismodictya prismatica, Hall (sp.). 



Page 83. 

 (See Plates XVII and LXII.) 



Figure 1. The apertural end of an average specimen; showing tlie slight 

 contraction toward the margin. 



Figure 2. A portion of a small and very slender example. 



Figure 3. A portion of an individual having a tendency to spiral gro^vth. 



Figure 4. A nearly entire individual of average size. 



Figure 5. An enlargement of the surface of an internal cast ; showing the 

 character of the reticulation and the rows of depressions on the 

 prism-angles at their intersection with the principal horizontal 

 spicular bands. Such pits probably represent the bases of spicular 

 tufts. 



Figure 6. Another internal cast also showing vertical rows of pits. 



Chemung group. Concord station, Erie county, Pennsylvania. 



Pkismodictya FiLiTExnus, Hall (sp.). 



Page 86. 

 (See Plate XXVII.) 



Figure 7. A gutta-percha squeeze of the original specimen which is an 

 external cast showing a fine reticulum and very obscure prism-faceSo 

 Chemung group Steuben county, New Yorh 



Pkismodictya pakallela, Hall (sp.). 



Page 82. 

 (See Plates XIX ami XXXV.) 



Figure 8. A fragment of a rather large, uncompressed specimen. 

 Figure 9. Transverse section of the same example. 

 Chemung group. Wellsville, New Yorh 



