22 ROBERT TRACY JACKSON ON ECHINI. 



One of the most interesting and instructive methods of preservation is when the surround- 

 ing matrix, both internal and external, is completely silicified and the original skeleton is entirely 

 gone, but is represented in finest details by molds in silica. Such an example is shown in Loven- 

 echinus missouriensis (Plate 39, figs. 4, 5). Here it is seen that everything that was not, is 

 represented bj^ the silica. This is rather a paradoxical statement, but fairly represents the 

 facts. The plates are not preserved, but the sutures between the plates are represented by 

 vertical siliceous ridges; the pores, both ambulacral and genital, are represented by vertical 

 tubes, which can be actually seen (Plate 43, fig. 5) passing from the proximal to the distal 

 side through the space which the plates originally occupied, but now simply a hollow. When 

 the external as well as the internal matrix is in hand (Plate 44, figs. 3, 4), we find on one side, 

 the internal matrix, a mold of the proximal side of the plates; on the other or external matrix 

 we get the distal mold of the plates showing spine tubercles and other external features. Sili- 

 ceous molds of the interior or exterior may be found in which there are no ridges representing 

 spaces between the plates, but the difference of conditions between these two types is not 

 understood. Molds, either internal or external, in fine sand or calcareous clay, are often ex- 

 tremely clear and may be accompanied by more or less of the shell of the test, or this may be 

 wanting. In a specimen of Palaeechinus quadriserialis in the British Museum, the test is 

 crystalline calcite, but only part of the plates are preserved; most of the rest are, however, 

 represented by impressions of their proximal faces on the matrix, as shown in Plate 29, fig. 1, 

 and Plate 30, figs. 1, 3. 



Sandstone molds may be so fine in details of preservation as to show the outline of plates, 

 tubercles, pores, and even peripodia. Internal sandstone molds show only impressions of the 

 proximal sides of plates with pores in relief as slight plugs. An excellent example is Hyatt- 

 echinus beecheri, a unique specimen (Plate 24, figs. 5-8), which shows the details of outline of 

 the proximal faces of the plates so clearly that I am enabled in Plate 2G to represent the sea- 

 urchin spread out, in which every plate was measured and the angles counted. External 

 sandstone molds are even better, for, on the exterior, details of structure are more strongly 

 marked; tubercles and peripodia exist as depressions (the impression being in reverse), and 

 ambulacral pores as slight elevated plugs. Examples are shown in Hyattechinus rarispinus 

 (Plate 23, fig. 1) and H. pentagonus (Plate 25, figs. 1-4). A curiously complex case is the 

 Hyattechinus rarispinus previously mentioned (Plate 23, fig. 1), in which is seen an external 

 mold of the ventral side, showing tubercles and peripodia, and in the same view an internal 

 mold of the dorsal side. The counterpart of this remarkable specimen (Plate 22, figs. 1, 2) 

 shows just the reverse condition; that is, an internal mold of the ventral side and an external 

 mold of the dorsal side. The test, which was extremely thin, is mostly wanting, but is existent 

 on the margins. The dorsal and ventral sides are brought almost in contact, yet the sandstone 

 did not close up, but retained as a thin interspace the vertical space originally occupied by 



