FROM THE PALAEOZOIC ROCKS OF INDIANA, KENTUCKY AND OHIO. 



447 



Interradial fields. — These arc five in number, of about seven pieces each. The first 

 piece rests in the angular notch between the first radials; these support two; those two 

 others upon the summit of the third range, in a notch between the pieces, rests the sixth 

 piece of the interradial fields; the sixth supports upon its upper oblique margin a 

 seventh piece, which rests also against the first piece of the second division of the rays. 

 There is some irregularity in this arrangement; some of the interradial fields carry the 

 seventh piece of the field with the double series, and close the series with a single piece ; 

 in these cases making eight the number of pieces forming the field. 



Lderhracldal fields. — Between the first divisions of the ray arc placed two pieces, one 

 above the other; they are entirely enclosed by the pieces forming the first and second 

 divisions of the rays. A single piece is placed above the second division of the rays, and 

 is enclosed by the pieces of the second and third division of the radials. 



The pieces composing the rays, interradial and interbrachial fields, are nearly equal in 

 size, and are so placed that they form entire zones around the body; nine such zones are 

 apparent in the fragment figured. Other fragments, from portions of the body near the 

 arms, show several more. 



The surface of the pieces forming the calyx arc variously ornamented, depending upon 

 the condition of the animal at the time it was buried. The best preserved specimens show 

 that the animal had a thick covering different from the bony skeleton, rising in well- 

 defined lines ; on this substance are bands of from six to two elevated ridges (most numer- 

 ous on the pieces near the base), that cross the body in every direction, crossing each other 

 at the centre of the pieces. Near the free rays the bands are reduced in number, and the 

 centre of the subdivisions of the rays are ornamented by an elevated ridge or carina, very 

 thick and high for the size of the pieces which they cover; the summit or back of the 

 carina seems to be plain or level; these ridges are slightly increased in width at the centre 

 of the pieces, which are crossed at right angles to the direction of the rays, by other carina 

 nearly equal in height to the first, and about half as wide as they are. In weathered or 

 worn specimens, that show the naked surface of the bone of the plates, the entire surface 

 of the pieces are covered with a network of fine ridges and depressions, having the general 

 direction of the ridges before described. The ridges frequently bifurcate and close again, 

 filling the surface of the pieces with irregularly-shaped pits or depressions. So varied are 

 the surface-markings srrowinsr out of the different conditions of the surface, that were sur- 



no o 



face-markings relied upon as a specific difference, many species would become necessary 

 for this species. 



found in this fragment bo carried to a complete circle, one hundred and twenty arm-bases of equal 

 size to those upon it, would be required to fill the circle. Many fragments have been observed that 

 are part of bodies nearly or quite one foot in diameter; this specimen is about four inches in diameter. 



