INTRODUCTION. 23 



the thickness of the test with some sand which filled the interior of the test, thus making an 

 internal mold possible. 



Loven devised the incomparable method of representing the structure of a sea-urchin 

 by a figure drawn from the ventral view and with the several ambulacral and interambulacral 

 areas spread out flat in one plane in a star-like fashion. Of course in the Palaeozoic it is only 

 rarely that one finds a fossil sufficiently perfect to admit of such treatment; but I give figures 

 drawn by this method of Bothriocidaris (Plate 1), Hyattechinus (Plate 26), Archaeocidaris 

 (Plate 10, fig. 10), Palaeechinus (Plate 30, fig. 3), Lovenechinus missouriensis (Plates 40, 41), 

 Lovenechinus sepiies (Plate 45), and Melonechinus (Plate 57). These genera are fairly repre- 

 sentative of the several groups of Palaeozoic Echini, and the figures show the characters of 

 the plates and the method of introduction of columns better than by any other method. 



Wliile perfect specimens are desirable and most highly prized, yet a great deal can be 

 made out of fragments when studied carefully. A left or right half of an ambulacrum repre- 

 sents the character of the whole. A piece out of the mid-zone, that is, halfway between the 

 mouth and middle of the periproct, represents the area where full specific characters are devel- 

 oped in both the ambulacrum and interambulacrum, and many species are represented by a 

 figure taken from the mid-zone (Plate 47). Such a figure does not show the development as 

 seen ventrally, or the apical disc, but these features are often wanting, or shown in related 

 species, so that a figure of an ambulacrum and interambulacnmi from the mid-zone gives for 

 these areas the essential specific criteria. 



The skeletons of Recent sea-urchins are often very fragile, and even if not so, the peristomal 

 and periproctal plates are easily injured and lost. It has been found a great help to diji the 

 specimens in, or brush them over with, a dilute solution of shellac in alcohol, or gelatine 

 dissolved in water. Specimens so treated are firm and solid and will stand even rough treat- 

 ment. A specimen with the spines all in place may be soaked in this manner by dipping, 

 and on account of the porosity of the skeleton, there will be no external evidence of the treat- 

 ment; the spines will be firmly fixed in place, and the specimen is in much safer condition for 

 storage or study. Where sutures are difficult to see, as in minute recent material, or ambu- 

 lacral details, it has been found a great help to wet them with benzole. When drying out, 

 sutures stand out clearly that are nearly or quite unrecognizable in the dry state. In prepar- 

 ing fresh material of Echini, I have found that a satisfactory method is to soak the specimens 

 over night in a considerable volume of fresh water to remove the salt, then immerse for a few 

 minutes in boiling water containing a liberal amount of corrosive sublimate in solution. This 

 treatment coagulates the albumen, poisons the specimens, and does not affect the color. They 

 will dry out without odor, and in excellent condition as museum specimens. The method has 

 the advantage of cheapness and quickness. When specimens are to be preserved in alcohol, 

 the treatment with fresh water is also desirable, as it kills the animal nicely relaxed, besides 

 removing much of the salt. 



