542 The American Naturalist. [June, 
ilarity in temperature, similar conditions of sedimentation, 
similar oceanic currents, and probably similar depths of water. 
As time passes and we ascend the geological scale, the num- 
ber and variety of genera of sponges increases. In rocks of 
Trenton age there has been found in a few localities in Ken- 
tucky, a form known as Brachiospongia (Fig, 5). It hasa large, 
cup shaped body, with an open, central cavity, and with from 
seven to thirteen arms radiating from it. No perfect speci- 
men of this has ever been found, and the conditions of preser- 
vation have not been such as to favor the presence of the 
minuter features. The probabilities are that it grew on the 
ocean floor, fastened. by a single point, with the open mouth 
of the cup above, and the so-called arms extending out in all 
directions. Its cruciform spicules ally it to certain modern 
living forms. 
In rocks of later age occur interesting cylindrical or turbin- 
ate forms described originally as sea-weeds. The framework 
is in the form of a net with regular meshes, the threads cross- 
ing each other at right angles. Professor Whitfield says that 
the threads “are not interwoven with 
each other like basket work, or like the 
fibres of cloth, nor do they unite with 
each other as do vegetable substances; 
but one set appears to pass on the out- 
side, and the other on the inside of the 
à body. The threads composing the net- 
as se Bruhn work vary in strength, and are in reg- 
ular sets in both directions.” 
One of the species of this family, known as Cyathophycus, 
occurs in clusters in the Utica slate rocks of New York. It is 
almost the earliest representative of a group that, in Devonian 
time, assumed a great development, and appeared in many 
different forms. The family is known as Dictyospongidx, and 
presents an interesting instance of the beginning, the culmina- 
tion and the dying out of a family of organisms. Beginning 
with the simple sac-shaped Cyathophycus (Fig. 6), or the glob- 
ular Rhombodictyon in the Utica slate, it branched off into 
prismatic, nodulose (Fig. 7) or spinose Dictyophytons in the 
