_-- THE PALEONTOLOGY OF MLJTKESOTA 



[Ataooporella. 



character is the development and continuous presence of cystiphragms in the axial 

 region of the zoarium. In this respect the species agrees with M. arborea and M. 

 gnuidis, and diners from all species of Homotrypa in which these structures should be 

 developed in the peripheral region only. Even granting the importance of this 

 distinction it should be understood that the present arrangement of the species 

 is regarded as provisional. 



Compared with M. arborea, the species under consideration is distinguished by 

 having larger zoeecia and thinner walls. The internal differences will be appreci- 

 ated at once in comparing the various figures of the two species given on plate XX. 

 Htmetrypa aillosn. illustrated on the same plate, has thicker walls, fewer diaphragms 

 and no cystiphragms in the axial region. 



feroKty. Rare in the Galena shales, i 



Genns ATACTOPOB LLLA. Ulrica. 



(partO, ULRJCH, 1*79. Jwr. CSn. S.v. Nat. Hist. *.. p. lift. 



FLIUCH. 18SS.Jour.C3n. Soc. Kat. HfcU wL TU p. 5C: HBO, GaL Starr. IE. * 

 p. SJO. 



(part.), NICHOLSON*. I88L "Tm*Qe5 Xtwticatipan,* p. SB. 



Zoarium generally forming thin crust? over foreign bodies, rarely lobate or sub- 

 Swtface commonly with monticules. Zoo^iawiUi very thin inflected walls, 

 their aueminius irregularly petaloid ; internally with cystiphragms. Mesopores angu- 

 lar, numerous, often completely isolating the xmecia : at first open and distinctly 

 tabulated, hot, when folly matured, largely or entirely filled by a deposit of srie^n 

 chyma. Acanthopores very numeroos, varying in sixe with the S]iecie& 

 ore or less upon the moectal cavity. 



TW agaities of this genus are with M**littif*r* qn the erne sMe and IVi tmtftra 

 v*. the other. Prom the first it is distinguished by the tbmthat development of 

 mMOpMUb and the more numerous aeaathopores. The* are not only usually 

 smaller in Jforfknttpw^ but they aho do not inftoct the notcial walk. IB Forw. 

 f*m> as restricted hy me, the mrimm is bifoliate, and UM mesonwres are net filled 

 up with age in the manner charattenstic of Atorttftntt*. 



Time may be in all about twelve species of this genus knowm to MK\ sis of 

 which are described from the Q wianati rte in the f*pers eOed afcawe. Tnesame 

 series of beds aford at least two new forn^ *n* tfe Ttenton of Kentncfcy and Ten- 

 while tW BMbeye and Trentoa ot Minnesota add the following 



