320 A FRESH WATER SPONGE FROM SABLE ISLAND.—MACKAY. 
the Geological Survey of Canada, in the fresh water pond found 
in the centre of that great sand-shoal in the Atlantic Ocean, 
well known as Sable Island, nearly one hundred miles from 
Nova Scotia, the nearest part of the continent. It was growing 
around the submerged portion of the slender stems of Myri- 
ophyllum tenellum, Bigelow, in green, compact, lobular masses, 
showing, where broken, numeroas orange yellow gemmules. 
It appears to approach most nearly to the following fresh 
water sponges described by Potts: [Meteromeyenia ryderi, v. 
baleni, found from Florida to New Jersey, in its spiculation ; 
and Heteromeyenia rydert v. walshir, from Gilder Pond, Massa- 
chusetts, in the fasciculation of its skeleton spicules. 
General form: Encrusting the submerged stems of Myri- 
ophyllum tenellum, (which in the specimens examined are about 
2 mm. thick), in a smooth, compact, green, lobular mass extend- 
ing toa gross diameter of about one centimeter, and to a height 
or length along the stem of about 5 or 6 centimeters in some 
cases, the lobes suggesting an abortive attempt at branching; 
pores and osteoles very minute; gemmules very abundant, 
appearing wherever the central mass is broken. 
Gemmules: Light orange in color, spherical, varying from 
500 to 800 microns in diameter, but generally between 600 to 
700 microns; foraminal aperture from 30 to 70 microns in 
diameter, not produced into a tube beyond the crust; dense 
inner (chitinous) coat of gemmule nearly 10 microns thick, 
surrounded by the light cellular crust (in which the short 
siliceous birotules or amphidisks are vertically embedded) toa 
depth of about 25 microns; both short and long birotules or 
amphidisks with one disk or rotule resting on the chitinous coat, 
their shafts radially directed, packed as closely as their disks 
allow, the long birotules being fewer with the distal rotules 
extending beyond the crust, their few slightly incurved rays 
somewhat adapted for attaching the gemmule to any finely 
fibrous environment. 
Short birotules: From 18 to 26 microns in length, 
generally from 20 to 24, with a smooth uniform shaft 
