Die Silurische Fauna den Wcstlichen Tennessee. ' 249 



generally from 2^"' to 3'" from the extremity of one ray to that of its 

 corresponding opposite one ; yet there are some that are smaller, and 

 some appreciably larger. The diameter of the rays is generally about 

 V". However, when the star is very small, the diameter is also 

 smaller, being at times not much more than a hair's breadth. Not sel- 

 dom, too, there are between the larger stars smaller ones of the same 

 forito, with thinner rays, and they are also easily distinguished from the 

 former, by a lighter color and a semi-transparency. In this case it 

 would seem, upon the whole, as if the larger stars were formed by the 

 incrustation of the smaller sort. Indeed, this formation of the larger 

 stars seems to be very probable, in some specimens, in which the upper 

 surface presents only a few stars, with very heavy and thick rays, but 

 in which the entire remaining surface is covered with irregular round 

 protuberances or tubercles, which only occasionally are grouped into 

 indistinct stars. At times, too, the rays of the stars are crossed length- 

 wise by a furrow, but this would appear to be only the effect of disin- 

 tegration. 



In regard to the arrangement of the stars no regularity can be dis- 

 covered. On the upper depressed surface, however, where they 

 appear most numerous and distinct, they always lie in a position par- 

 allel with that surface. Beneath those that are nearest to the upper 

 surface other stars are visible, which also present, in respect to those 

 lying above them, no regularity, but are entirely covered by them, 

 without any order whatever. As a rule, the stars on the lower convex 

 side of the body are much less distinct and numerous than on the upper 

 side. Ordinarily but few separate and indistinct traces are to be seen 

 of them on the latter side. Yet, there are specimens in which the 

 stars appear as perfect and numerous on the lower as on the upper side. 

 In such cases they are often very plainly discernible on the inclined or 

 almost perpendicular side surface of the lower half, and lie in a plane 

 paiallel with the outer side. From disintegrated or fractured speci- 

 mens, it is clearly to be seen that the stars are also distributed through- 

 out the inner mass of the body. The spaces between the stars, 

 throughout the whole thickness of the body, are filled up with a homo- 

 genous fossil substance, in which no further organic structure can be 

 discerned. In one of the specimens before us there is a coarse fibrous, 

 or thin, prismatic separation (or disconnection) visible, so shaped that 

 the fibers or their prisms nui almost perpendicularly to the upper con- 

 cave surface. 



The substance of the fossil is in contrast with the structure of the 



sponges previously described, from the fact that in this genus a light 



,gray calcareous matter predominates. The stars especially are thus 



