1887.] NATURAL SCIENCES OF PHILADELPHIA. 213 



Birotulate spicules surrounding the gemmules very short, umbo- 

 nate ; rotules sometimes twisted or contorted ; margins entire ; genei*- 

 ally exflected or turned up saucer-like, away from the surface of the 

 gemmule ; the diameter of the outer rotule generally rather less 

 than that of the proximal one. (PL X, fig. i, c,d, etc.) 



Meas. Skeleton spicules 0*00466 by 0*00045 inches. Length of 

 birotulates 0*00045 inches. Diameter of large rotule 0*00055 inches, 

 and of shaft 0*0001 inches. 



Hub. Encrusting timbers and stone Avork at various, probably 

 sometimes at great, depths. 



Loc. Found as yet only in the Schuylkill River and reservoirs 

 near Philadelphia; at Windmill Island in the Delaw'are River; and 

 near Phillipsburg, New Jersey. 



Several circumstances give to this species of fresh water sponge 

 peculiar interest and importance. Described at first from a speci- 

 men inadvertently sent by Prof Leidy to Dr. Bowerbank, as men- 

 tioned by the latter in his "Monograph," (Proc. Zool. Soc. 1863, p. 

 445) and already alluded to in my historical sketch of Spongilla 

 fragilis, it appears to have remained unseen and uncollected from 

 1863 until 1880 when the writer rediscovered it at the original local- 

 ity ,-the neighborhood of Fairmount Dam on the Schuylkill River, 

 Philadelphia. I am not aware that any has been found elsewhere, 

 excepting at Windmill Island in the Delaware River, near by, and a 

 single dry specimen, origin unknown, upon a stick which I picked up 

 upon the bank, high above the Delaware River at Phillipsburg, New 

 Jersey. 



Fairmount Dam, just mentioned and frequently referred to in 

 these pages, is, of course, already known to Philadelphians ; but as 

 it has been spoken of as one of the richest localities in the world 

 for fresh water sponges, a brief description of the situation may be 

 allowed, for the information of others. It is situated at the head of 

 tide water in the Schuylkill River within the limits of the City of 

 Philadelphia. It supplies upon one side, a system of locks pertain- 

 ing to the Schuylkill Navigation Co. and on the other, pours its living 

 floods through the magnificent turbine water wheels, by which the 

 pumps are operated, that raise a large portion of the water-supply 

 of the City to the subsiding and distributing reservoirs upon the 

 summit of the neighboring Fairmount Hill. The direction of the 

 dam breast is not at right angles to the course of the stream, but 

 follows a line of rocks diagonally toward the northwest, turning 



