86 The Ottawa Naturalist. [November 



Beneath the thin non-porous epistereom lies the thick mesostereom. 

 That part of the mesostereom which is in contact with the epistereom 

 forms a practically continuous sheet, penetrated only by pores, and 

 from this sheet the greater part of the mesostereom is suspended in the 

 form of vertical lamellae. (Plate IV, figs. 3 and ID.) Viewed along 

 the suture planes, where exposed by the dismembering of the theca, 

 these lamellae appear thin and narrow toward their junction with the 

 continuous exterior part of the mesostereom, but they thicken toward 

 their inner terminations for a distance of almost a millimeter. These 

 lamellae do not radiate from the center of the thecal plates, but form 

 groups, all lamellae belonging to the same group being perpendicular 

 to the same suture line between two adjacent plates. If imaginery lines 

 be drawn from the center to the angles of each plate, then the lamellae 

 will be found grouped in triangles limited laterally by these imaginary 

 lines. In each triangle the lamellae will be found perpendicular to the 

 suture line forming the base of the triangle, the triangles of adjacent 

 plates forming rhombs, which, however, give no indication of their 

 presence on the un weathered surface of the plates. The adjacent 

 triangular groups of lamellae are separated usually by grooves, widen- 

 ing toward the center of the plates and narrowing toward the angles. 

 Both the lamellae and the inter-lamellar spaces are directly connected 

 across the suture planes. 



The epistereom is thin and non-porous. However, if only slightly 

 weathered, it is found to be underlaid by pairs of short lunate pores 

 extending parallel to the epistereom, just beneath the latter, appearing 

 on the weathered upper surface of the mesostereom as short lunate 

 grooves, the concave sides of each pair facing each other. The pre- 

 sence of these pairs of lunate pores often is indicated on the exterior 

 surface of the epistereom by short lunate ridges (Plate II, figs. 1A, IB, 

 also ID), which correspond in size, form and position with the pores 

 beneath. Three or four series of these pairs of lunate pores may 

 occur between the centers of the thecal plates and the suture lines, the 

 pairs of different series more or less alternating with each other in 

 position. 



Each lunate pore is connected near its distal end with a small 

 circular or oblong pore penetrating the outer more or less continuous 

 sheet of the mesostereom, and leading into the spaces between the 

 vertical lamellae. Pores of the same pair always connect with different 

 inter-lamellar spaces, being separated by one of the lamellae. The 

 right hand pore of one pair, however, usually is connected with the 

 same inter-lamellar space as the left hand pore of the nearest adjacent 

 pair, proximally or distally, i.e., either nearer the center of the thecal 

 plate or nearer the suture line. In this manner, three or four pores 

 belonging to different pairs may be connected to the same inter-lamellar 

 space. There is no connection between pores belonging to the same 

 pair. 



