CLIXTOX. L(JWEK IlELDEHBEHC;. CIIEMUXG. AND WAVEULV GROUPS. 459 



The deep tuirows on the ri^ht and left sides of the shell ap[)ear to be constant fea- 

 tures, the others are somewhat variable. 



Aperture broadly oval, nearly circular ; peristome concealed on all the speci- 

 mens observed, so that its true form can not be ascertained. 



All the specimens occurrinj:,- in the form of casts, the surface markings are 

 obsolete. 



This species may be disting-uished from Plalyceras mitelliformis by its straighter 

 form, absence of a prominent carina, and by its conspicuous plications; from P. (0.) 

 stn'atuin by its smaller si/e, more frequent i)lications, and the absence of radiating 

 stria', the convexity of the anterior side and the concavit}' of the posterior ; from 

 Platijceras incqnale by its equally developed right and left sides and its conspicuous 

 plications; from P. (0.) breve by its curved, more slender form, and plications; 

 from Phttyceras dwsale by its smaller size and conspicuous plications; from P. (0.) 

 attenuntum by the straight apex and the strong lateral furrows ; from Plalyceras 

 carinutnm by the absence of a prominent carina and by the conspicuous lateral furi-ows ; 

 from P. (0.) com'cum by its smaller size and curved form. 



Forviotion unci locality. Chemung grouj). four miles north-west of Warren. "War- 

 ren county, I'cnn'iylvania. 



ACERVULARIA COMMUNIS Simpson, n. sp., Fig. 20. 



Growing in large convex or globular masses ; under side covered by a wrinkled 

 epithecal crust. Stems multiplying by gemmation from the calyces; usually in con- 

 tact, but sometimes free and circular. vi. 



Calyces in contact, polygonal, generally pentagonal or he.\-ago- 

 nal ; surrounded by acute, linear, crested edges ; diameter a little 

 variable, but usually about 5 mm. Sides of calyces al)ruptl\' curved 

 downward to the central cavity. ^ps Tr.i889 



Jiays thin, sharp; about thirty in nuinljcr; alternately long and short, but of 

 uniform size at the margins of the calyces. Diai^hragms thin. 



Formation and locality. Lower Helderberg group, from .\. 15. Miller's farm. 

 Warrior lidge, Barree township, Huntingdon county, Pennsylvania. 



CLADOPORA RF:CTILTNKATA Simpson, n. sp., Fig. .30. 



Zoai'ium I'ainose, l)il'urcating, rarely anastomosing ; diamctci- of Itranches fi-om 

 l.JU to 3 mm. 



