INTRODUCTION. 21 



right to left, as Hyatkchinus rarispinus (Plate 23, fig. 3) and H. pentagonus (Plate 25, fig. 3). 

 In molds of the exterior, if ventral : the axes revolve clockwise, from left to right, as in Hrjatt- 

 echinus rarispinus (Plate 23, fig. 1) and H. pentagonus (Plate 25, fig. 1). Orientation, not only 

 for the sake of the axes, but also for the correct position and introduction of columns of plates 

 and direction of imbrication, must be borne in mind in a reversed specimen; for naturally 

 confusion results unless this is taken into consideration. It was a strong temptation to reverse 

 the drawings, so that external molds seen from the interior might appear as if seen from the 

 exterior. It would have simplified matters as regards orientation, but then the drawings 

 would not have corresponded with photographic figures of the same specimen ; also the com- 

 plication is so great that I feared to make errors by reversing drawings. 



The terms molds and casts are often used indifferently, but it is necessary to distinguii^h 

 them. A mold, either internal or external, is an impression of a fossil and represents the object 

 in reverse. A cast, either natural or artificial, is a mechanical filling of the mold and represents 

 therefore a replica of the original. A pseudomorph differs from a cast simply in that it is a 

 molecular chemical replacement of the original, as in silicification, instead of a mechanical 

 filling of a mold, as in the case of mud, sand, or plaster. Most fossils, when they are not the 

 original skeleton, are either pseudomorphs or molds; natural casts, strictly speaking, are com- 

 paratively infrequent. 



In the figure of H yattechinus rarispinus (Plate 23, fig. 1) the test is wanting, and the view 

 represents an external sandstone mold of the ventral side, together with an internal mold of the 

 dorsal side, seen from above. The mold of the ventral side, being external, shows the tubercles 

 and peripodia of the ambulacral pores, but the mold of the dorsal side, being an impression 

 of the interior, has no tubercles, and the ambulacral pores are represented by vertical plugs. 

 This shows that the point of view must be constantly borne in mind and orientation carefully 

 considered. As an aid in orientation, a rubber ball was marked with ambulacral and inter- 

 ambulacral areas, then cut down on these lines, and marked within. Also dorsal and ventral 

 plaster molds of a Cidaris were made and lettered as models, for it was found difficult to keep 

 orientation clearly in mind in reversed views, as are external molds. 



An interesting matter is the condition of preservation of Palaeozoic or other fossil Echini. 

 I have never seen a fossil sea-urchin in which the test was preserved in its original condition, 

 but rather some chemical change seems always to have taken place even in late Tertiary speci- 

 mens. In other groups of animals, as fossil molluscs, corals, etc., the original skeleton is often 

 preserved without any change except the leaching out of organic matter. The sea-urchin 

 skeleton is composed of such loose network-like structure, that it seems to be peculiarly open to 

 chemical readjustment and change. When the skeleton is calcified, the original microscopic ' 

 structure is, at least usually, quite destroyed. The skeleton is often replaced by silica, when we 

 may get very beautiful pseudomorphs preserving the finest details; an example of this is 

 Melonechinus giganteus (Plate 60, fig. 3). 



