12 FREDERICK IV. SARDESON 



presumably built into corresponding invaginations of the living 

 cortex. They are not cells or pores, but have been supposed to 

 be, hence the name. They are inconspicuous in this species or 

 even wanting, and will be discussed later. 



Characters sufficient for the determination are upon the 

 exterior in this as in related species. The peculiar cell pattern 

 can be readily distinguished from structurally very similar ones. 

 The variability in cells, acanthopores, etc., will however be found 

 greater, and there are fewer true species of the genus than 

 recorded. Other species of Pvasopora have more perigene 

 growth, or, again, acrogene ones are the Monticulapora. 



Homotrypa minnesotensis Ulr. has long, round, slowly-branch- 

 ing zoarium, with many specific marks as to cells, monticules, 

 etc. Fossil stems are in part or entirely hollow, because the 

 axial region has very thin crenulous walls without tabulae, hence 

 the sea water could enter and eat away the walls, leaving, if 

 anything, the thicker-walled tabulate peripheral region. The 

 peripheral cell has cystiphrams similar to Prasopora. 



Batostoma fertile Ulr. consists of large, flattened, or round, 

 somewhat irregularly branching acrogene parts, arising from a 

 large basal expansion of irregular perigene growth. A single 

 basal may support more than one or, again, no acrogene part ; but 

 as in Callopora a thick walled maturity follows the thin walled, 

 immature stage. Thus the characters of peripheral and axial 

 regions are evident when there is even no acrogene growth. 

 Omitting some details, the peripheral region has thick walls on 

 which are well-developed spines, acanthopores. Mesopores may 

 be very few or, again, very numerous on parts of the same 

 specimen (PI. A, Fig. io,a,b) . They are "closed," i.e., their tab- 

 ulae built close up to the apertural margin, by which the meso- 

 pores, being shallow or confluent, look like closed interspaces 

 merely between the rounded autocells. The cell clusters, which 

 are in place of monticules, have macules, i. e., clusters of meso- 

 pores at their center. The young cells of the axial region 



