DIAGNOSTIC FEATURES OF ALLIED GENERA. 270 



this genus from ChonophyUum* Nearly or quite as much twisting, how- 

 ever, may occur in 0. ponderosum, so that this character alone cannot be 

 relied upon for their separation. No spicules or supporting growths are 

 found in these type forms. Distinct and, at times, strong tabula? are 

 found in the central area. The layers curve downward between the septal 

 ridges and form coarse, subvesicular structures in the outer interseptal 

 cavities, just as in Omphyma. The broad bands upon the epitheca cor- 

 respond to the interseptal cavities and the fine, longitudinal grooves mark 

 the position of the septa. 



The following details of structure will ordinarily serve for its complete 

 separation from ChonophyUum: 



1. The more persistent epitheca and occasional radiciform process. 



2. The well developed tabulae through the central area. 



•'!. Cell-cups forming sharp or angular septa by their radial infoldings. 



4. Absence of supporting growths. 



•">. False columella and common elevation of the bottom of the calyx. 



(>. The coarse subvesicular structure of the interseptal cavities. 



7. The generally distinct fovea. 



8. The broad longitudinal bands upon the epitheca representing the 

 interseptal cavities. 



We have met a collection of some 7 or 8 specimens from the Helder- 

 berg group, of Kentucky, 40 miles south of Louisville, in which the 

 characters of these two genera are to some extent combined. They have 

 an irregular Ptychophyllum growth, epitheca well preserved, a spongy 

 columella projecting from the bottom of the pit, one well defined fovea 

 and, occasionally, radiciform processes. ( )n the other hand, the septa, as 

 ^■<-n in the outer area of the calyx, are not angular but rounded and 

 convex as in ChonophyUum, and the broad longitudinal bands mark the 

 position of the septa. Unfortunately the specimens are so solidly silici- 

 fied that but little of their actual structure can be made out. Basing our 

 judgment simply upon the external characters, we prefer to assign these 

 forms to PtychophyUum. 



Cyathophyllum, Goldfuss,1826. 



The simple forms of this genus are usually readily separated from 

 Ch&nophyllum by the lamellar septa and the development of tabulae. 

 Simple forms, however, of the species still commonly known as Cyatho- 

 phyllum helianthoides, Goldf., are more closely related, ami it may he well 

 to separate the two, qow that a form of ( 'honophyllum is known so similar 

 in general appearance. These are turbinate, with regularly and strongly 



* Bril Foss, < lor., pt. i, L850, p. I six. 



