128 LOWER PENINSULA. 



thirty-six to forty. Lamellae linear within the inner cycle, and tent- 

 shaped on the expanded peripheral portions of the calycinal conflu- 

 ent disks. Such tent-form is evidently not caused by any divergence 

 of the two leaves composing the crests, but by the oblique lateral 

 attachment of the interstitial vesicles to them. The crenulations 

 of the edges of the lamellae have the form of linear crossbars. The 

 centre of the calyx bottom is raised into a columellar knot, and in 

 vertical sections of calcified specimens a central string of greater 

 density can be observed, but it is not a solid axal column ; in some 

 specimens no indication of a columella is perceptible. 



Grows in large discoid or convex masses, and is found frequently 

 in the drift deposits of Michigan, associated with fossils of the cor- 

 niferous limestone. 



Plate XXXVIII. — Fig. 2 represents two silicified fragments 

 found in the drift of Ann Arbor. 



PHILLIPSASTR.'EA GIGAS (?) Owen. 

 PHILLIPSASTR^A GIGAS, Billings. 



Large lenticular masses covered by an epitheca on the lower 

 side ; upper side formed by large confluent star cells perfectly 

 resembling the smaller form, Pliillipsast¥^]^crnciiilli. The polyp cells 

 are unequal, owing to the frequent intercalation of young calyces. 

 The cells are not perfectly confluent with their horizontally ex- 

 planate margins ; an obscure polygonal defining line indicates the 

 extent of every individual calyx, but an intermediate wall is not 

 developed. Diameter of the larger calyces over two centimeters ; 

 diameter of inner cell pit not quite one centimeter. The cir- 

 cumference of the inner cell pit is raised into a rounded monticulose 

 rim. Bottom of inner cell pit reversed into a small central boss, on 

 which the lamelhe unite with a spiral twist. Circumference of cells 

 radiated by from fifty to sixty crenulated lamellae. 



P'ound in the upper strata of the Island of Mackinac. 



Plate XXX VIL, Lower row. — The left-hand figure presents a 

 surface view of a calcified specimen from Mackinac. This form is 

 in all particulars, except in the size of the calyces, similar to 



