250 Die SiluriscJie Fauna des Westlichen Tennessee. 



composed, A fracture of the pieces generally presents the leafy struc- 

 ture of calc-spar. In the middle part, however, there is commonly a 

 nucleus of dark horn-stone impregnated with calc-spar discernible, and, 

 at times, silicification also pervades the greater portion of the inner 

 mass. 



In respect to the outer form of the specimens, it is, on the whole, 

 very commonly globular ; without being, however, altogether devoid 

 of certain deviations and irregularities. The upper surface is often 

 only slightly depressed and almost level, and in other cases very much 

 hollowed and almost of a cup form. In the latter case, the bottom part 

 is of unusual thickness, and the entire appearance is that of a coarse 

 cup. Only one specimen presents an entirely irregular form ; it has 

 the appearance of having been jD^essed together at the sides, and on 

 ■ one side is prolonged into a grooved formed continuation. The whole 

 form of this sjDecimen, which appears to have been compressed between 

 foreign bodies in the development of its regular forms, leads us to the 

 conclusion that the animal, in life, was possessed of a certain degree of 

 flexibility or j^lasticity. Not one of the numerous specimens before us 

 presented the faintest trace of an epithica. The body, when alive, was 

 manifestly unattached, and lay on the bed of the sea with its concave 

 side turned upward. 



In respect to the zoological relationship of this body, the general 

 form, which no other class of animals possess, as well as its appearance, 

 would indicate that it is connected with the sponges, but in many 

 other respects this species would appear to be wholly peculiar. What 

 -vs, first of all, the meaning of the star-shaped bodies? They can be 

 nothing other than spiadcc, or silicious needles. Yet tTiey have never 

 been found in any living or fossil sponges, in such regular star-shaped 

 groups or in such magnitude. Moreover, the calcareous nature of the 

 fossilized substance is remai'kable. Most fossil sponges, and especially 

 all the other species wdiich occur in the same strata, are silicious. 

 Here, on the contrary, the fossil is almost entirely calcareous. This 

 proves beyond a doubt that the chemical composition of the living 

 body was essentially different from that of others occurring with it, • 

 and Avhich are always complete silicious sponges. This species belongs 

 apparently to the calcareous sponges, which, as Professor Steenstrup, 

 of Copenhagen, has shown, are frequently found in the seas of the 

 present epoch, near the sponges in which silica predominates. 

 Through the courtesy of M. Steenstrup, I have in my possession a 

 specimen of an undescribed species of the calcareous sponges of Green- 

 land, which contains three-pointed spicules, plainly visible to the' 

 naked eye, and easily dissolved in muriatic acid. This sj)ecies of the 



