120 LOWER PENINSULA. 



surface to the centre. There are three different modifications in 

 the structure. In one the demarkation of an inner and an outer 

 area is very obscure ; the vertical lamellae reach to the centre of 

 the diaphragms as superficial carinae. These are exclusively 

 Silurian forms, which might be distinguished as a peculiar generic 

 type, but as their general mode of growth is so similar to the other 

 biareal forms of Diphyphyllum, I prefer to leave them together. 

 A second modification has a very broad central area, formed almost 

 exclusively by transverse diaphragms, while the vertical lamellae 

 are confined to a narrow peripheral cycle ; but the inner and outer 

 cycle is not defined by an intermediate internal vertical wall. The 

 third modification, which is generally of stouter growth than the 

 second form, has the inner area defined from the outer by a dis- 

 tinct vertical wall of horseshoe shape, open on the side of the 

 apertural fovea. The lamellae in this latter form never transgress 

 the inner wall, and the central part within is exclusively formed 

 by a superimposed series of transverse diaphragmatic plates. 



DIPHYPHYLLUM HURONICUM, N. Sp. 



Aggregated, cylindrical, flexuose stems of a diameter of from one 

 to two centimeters, annulated by delicate striae of growth, and by 

 deeper wrinkles and constrictions ; longitudinally ribbed by septal 

 furrows. The stems are laterally connected by stout rugose prolong- 

 ations from their walls, which in all the stems of a colony are uni- 

 formly directed to one side. Calyces moderately deep, dish-shaped, 

 with explanate margins, radiated by about sixty linear lamella:, 

 which unite in a central fascicle ; their margins are faintly cren- 

 ulated, or not so at all. Interstices filled with vesiculose plates, 

 which in the central part are larger and inclose somewhat coarser 

 cell spaces, but no distinct large plates, properly deserving the 

 name of diaphragms, are developed, and the outer and inner area 

 are not well defined. The stems multipl}' by gemmation from the 

 centre and the margins of the end cells, the marginal gemma; re- 

 maining for a good while of a more slender, smaller size than the 

 contemporaneous new cells sprouting from the centre. Found in 

 the Niagara group of Drummond's Island and Point Detour. 



Plate XLV. — P'ig. i represents a side view of a cluster of stems 

 found at Point Detour. 



