150 NEW YORK STATE MUSEUM 



sections, less frequently on the others ; but all of which indicate Silur- 

 ian age. In one or two cases dermal spicules are grouped in a common 

 base, yet so minute that the spicule with its rootlet measures but 

 0.0 1 mm in length and the fragment in which it is embedded only 

 0.03 mm in outside diameter. 



These spicules belong to Lithistidae and are similar to those of 

 Anomalospongia reticulata Ulrich from the Cincinnati 

 horizon in Kentucky, but the latter are many times larger than our 

 specimens. They show below the prongs the widely divergent 

 angle of about no degrees which pertains to the four-rayed type. 

 This serves to separate these from six-rayed hexactinellid spicules 

 whose cross sections also show three-rayed structure. These num- 

 erous uniaxial and tetraxial spicules are quite typical of both lower 

 and upper Silurian. The massive thick-walled, siliceous paleozoic 

 sponges belonging to the order Lithistidae occur from the Cambrian 

 to the Recent but become more abundant in Silurian strata, then 

 again in the Jura and Cretaceous. Some of these sponges may have 

 furnished the silica in which the Foraminifera were embedded; and 

 none of the full sponge masses remain. On slide 6 is a fine fragment 

 of a minute sponge with six tetraxial spicules of dermal character 

 but only five show the forking terminal. They project above the 

 sarcode base 0,01 mm and are remarkable for their delicate structure. 

 Only in one case did we detect a club-shaped spicule of triangular 

 shape and it is possible that this is a rhabdolith.^ We have not the 

 literature to study fully these sponges but consider from the evidence 

 at hand that they are of Silurian age, perhaps Upper Silurian in some 

 cases. It is not improbable, however, that they come chiefly from 

 the upper Ordovician as sponges of this type range well through the 

 entire Silurian period. 



Evidence from Radiolaria 



The second group of organisms represented on these slides is 

 Radiolaria. These are mostly fragments of coarsely perforate 

 spherical shells and in no case do we find complete forms which admit 

 identification. The best example we discover is seen on slide 4, 

 This radiolarian has a spherical test with elongated axial rays, four 

 of which can easily be made out. These come out from the disc 

 without enlargement, as in Lichnaspis and not very different from 

 Acanthometra, the latter possessing twenty such spinous processes. 

 The spherical fragments resemble Cenosphaera macropora 



^The Microscopical Fauna of the Cretaceous of Minnesota, Woodward & 

 Thomas, Geol. Minn., v. 3, pt i, 1895, p. 49, pi. E, fig. 2. 



