272 THE PALEONTOLOGY OF MINNESOTA. 



[Dekayella prsBnuntia. 



with the two varieties immediately preceding, but specimens are distinguished at 

 once by the much greater size of the acanthopores, these being so large and prom- 

 inent that they are easily seen by the unassisted eye. An average distance of 0.5 mm. 

 separates them. Internally the walls vary in thickness, very commonly even in the 

 same tangential section. Where they are thinnest the acanthopores are the most 

 distinct, and it is chiefly in such parts that the small set is determinable. Aside 

 from the latter the general appearance of tangential sections is decidedly like that 

 of typical Dekayia. 



Mus. Reg. Nos. 6016, 7657, 8022, 8098. 



Var. MULTiPOEA, n. var. 



PLATE XXIII, FIGS. 44-47. 



In this form the acanthopores are likewise very large and, as a rule, are to be 

 seen with the naked eye on the surface of all well preserved specimens. Still there is, 

 as may be seen by comparing figs. 44 and 45, considerable variation in their size, so 

 that the identification of the variety depends chiefly upon the unusually numerous 

 and large mesopores and the rounded shape of the zooecia. In many cases, however, 

 it is difficult to discriminate positively between the two sets of tubes, as these are 

 shown in tangential sections. As a rule — perhaps the test is reliable at all times — 

 the zooecia never have any part of their walls convex upon the inner side. In 

 vertical sections the mesopores are distinguished by having the diaphragms more 

 crowded than they are in the zooecial tubes. A radial arrangement of tb.e cells 

 about the acanthopores, as shown in fig. 45, is frequently noticeable. Both sets of 

 acanthopores about equally numerous, but the smaller set is liable to be overlooked 

 except when the walls are unusually thin. 



Mus. Beg.'JNoa. 6021, 8306. 



These species and varieties gave me more trouble than the whole genus Homo- 

 trypa. It seemed impossible to draw up a thoroughly satisfactory classification of the 

 hundreds'of Minnesota specimens of Dekayella studied. The separations made were 

 generally recognizable, and some of them are based upon not only obvious, but upon 

 what, as a rule, we may regard as important structural deviations. Extended inves- 

 tigation, however, seemed to show that in the present cases the peculiarities were 

 too inconstant to deserve specific recognition. The var. simplex appears to be the 

 best marked and most constant, and should, perhaps, have been called a good species, 

 with var. ncevigera under it. In that case the var. echinata also should be raised to 



