THE FOSSIL CRINOID GENUS DOLATOCRINUS AND ITS ALLIES. 23 



Hat or concave base. Along with these various shapes, but not always 

 concurrent!}^ with particuhir forms, the tegmen may vary from 

 nearly flat to convex or conical; deeply lobed or sloping evenly; with 

 plates well defined, or rather obscure on the smooth surface; w4th 

 surface granular, covered with small pustules, sharp or rounded 

 nodes, or strong spines. The base may be sharply excavate into a 

 deep or shallow^ pit enclosing the column, broadly concave, flat, or 

 protuberant. In some forms the tegmen is remarkably constant 

 and characteristic; in others, especially the large bursiform type, it 

 is subject to much change due to pressure in fossilization. 



Surface ornament. — There are upon the plates of the dorsal cup 

 two kinds of sculpturing: (1) A longitudinal median ridge following 

 the radial series, which may be limited to a few of the lower plates 

 or may traverse the entire distance to the arm bases; may be low, 

 rounded, inconspicuous, or high, shiirp, and very prominent; or it 

 may be absent altogether. It is formed with nodes at the centers of 

 the plates as nuclei, which may be elongated until they connect at 

 the sutures, or may be connected by a rounded neck; if the connect- 

 ing Jieck approaches the size of the nodes, the ridge may be called 

 continuous, and if very much smaller and inconspicuous, it may be 

 called discontinuous. Agam, the central nodes may be substantially 

 isolated, but even then there is usually some trace of a connection 

 marking the course of the buried nerve cords. (2) A secondary 

 sculpturing which consists of more or less continuous radiating 

 striae, straight or wrinkled, or lines of small pustules, passing from 

 ])late to plate, tending to form concentric triangles; or of nodes or 

 pustules not radiately arranged. Primarily the ridges connecting 

 the centers of both radial and interradial sets of plates are the external 

 representatives of the nerve cords which innervate the growing 

 skeleton, and the triangular arrangement results mechanically from 

 the mutual relations of the plates. With age they may become 

 variously modified by secondary growth, reduplicated, intensified, 

 or broken up into nodes by w^hich the original fine lines are inter- 

 rupted, obscured, or obliterated. These processes will produce 

 extremes of the tw^o types of sculpture, between which there may be 

 an infinite number of combinations in which the tw^o are more or less 

 intermingled. Sometimes the striae are bent or wrinkled, making a 

 vermicular kind of ornament; and in some cases there are neither 

 definite striae nor nodes, but all plates are tumid and radiately fur- 

 row^ed. If certain lines of the triangles are accentuated, a stellate 

 figure is produced around the base. On account of intergradation 

 between the different tyjjes, too much importance must not be 

 attached to the surface ornament. Even Miller and Gurle}^ had 

 some doubt about it, for they said in Bulletin 4, page 25: 



We are satisfied the sculpturing is not uniform on specimens belonging to the 

 same species. 



