I.NTUODC* TION. 21 



right to left, as Hyattechinus rarispinus (Plate 23, fig. 3) ami //. i>entagonus (Hat,- _:,, fig. 3). 

 In molds of the exterior, if ventral: the axes revolve clockwise, from left to right, 11* in //////- 

 echinus rarispinus (Plate 23, fig. 1) and //. /*-,//,///// ( I'lute _>:>, fin. 1 1. Orientation. ,,,,i only 

 for the sake of the axes, but also for the correct position and introduction of columns .if plate* 

 and direction of imbrication, must be borne in mind in a rever-ed -prcim.-n; for naturally 

 confusion results unless this is taken into consideration. It *M a -trong temptation to n 

 the drawings, so that external molds scon from the interior might appear as if -em from tin- 

 exterior. It would have simplified matters as regards orientation, hut then tin- drawing- 

 would not have corresponded with photographic figures of the same -peciim-H: also the (.mi- 

 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 nece ary to distingui-h 

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

 in reverse. A cast, either natural or artificial, is a mechanical filling of the mold and n-pre-ent- 

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

 molecular chemical replacement of the original, as in silicification. in-tead of a mechanical 

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

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

 paratively infrequent. 



In the figure of Hyattechinus rarispinus (Plate 23, fig. 1) the te>t i- wanting, ami 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, -hows the tuberde- 

 and peripodia of the ambulacral pores, but the mold of the dorsal side, being an imprr>.-ion 

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

 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 fn>.-il Kchini. 

 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 s| M -ci- 

 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 ..|*-n 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 

 Melonechinus giganteus (Plate 60, fig. 3). 



