EAELY PALEOZOIC BKYOZOA OF THE BALTIC PEOVINCES. 



195 



mesopores hold approximately the same number of straight dia- 

 phragms. In deep tangential sections both the zooecia and meso- 

 pores are thin walled and polygonal, but as the surface is approached, 

 the zooBcial walls thicken at the expense of the mesopores. In the 

 outermost parts of the mature region, calcareous tissue is often devel- 

 oped in the mesopores to such an extent that the latter are obHterated. 

 Eight zooecia in 2 mm. 



Closely compared with typical Homotrypella Jiospitalis, variety 

 crassa differs but slightly and apparently only in having more 

 crowded diaphragms and cystiphragms and less developed acantho- 





Fig. 105.— Homotrypella hospitalis ceassa. a, a Russian example, natural size; 6, tangential 



SECTION, X40, through THE LESS MATURE PORTION OF A ZOCECIAL LAYER; C, TANGENTIAL SECTION, X40, 

 THROUGH THE MOST MATURE REGION OF THIS SPECIMEN, WHERE THE MESOPORES ARE OBSCURED BY A 

 DENSE tissue; A, e, and/, three VERTICAL SECTIONS, X20, ILLUSTRATING THE CHARACTERS IN VARIOUS 

 PARTS OF THE ZOARIUM. IN d THE YOUNGEST STAGE IS FIGURED WITH THE IMMATURE ZONE, AND THE 

 EARLIEST PART OF THE MATURE REGION DISPLAYED. WASSALEM BEDS (D3), UXNORM, ESTHONLi. 



pores. Although even these differences are subject to slight varia- 

 tion, it is beheved that a varietal name for the older form is justified. 



Occurrence. — The typical form is abundant and characteristic in 

 the Richmond formation at many American localities; the variety 

 crassa is known from the Black River shales of Minnesota, Iowa, and 

 elsewhere, and from the lower Trenton rocks, particularly at West 

 Covington, Kentucky. The Russian specimen was found in the 

 Wassalem beds (D3) at Uxnorm, near Reval, Esthonia. 



Plesiotij2)e.— Cat. No. 57279, U.S.N.M. 



Thin sections of the Russian type are in the collections of the 

 British Museum. 



