INTRODUCTION. 



the thickness of the test with some sand which filled the interior of tin- tot, thu- making an 

 internal mold possible. 



Loven devised the incomparable method of representing tin 1 structure of a nca-un-hiii 

 by a figure drawn from the ventral view and with the several ambulacra! and intcrambiilacral 

 areas spread out flat in one plane in a star-like fashion. Of cour-e in tin- Palaeo/oic it i- only 

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

 drawn by this method of Bothriocidaris (Plate 1), Hyattechinus i Plate _'',), Archaeoci.lari- 

 (Plate 10, fig. 10), Palaeechinus (Plate 30, fig. 3), Lovenechinus missouriensi* I'late- lo. 41), 

 Lovenechinus septies (Plate 45), and Melonechinus (Plate 57). The-e genera are fairly repre- 

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

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



While perfect specimens are desirable and most highly prized, yet a great deal can l>e 

 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 tin- 

 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 repn-ented by a 

 figure taken from the mid-zone (Plate 47). Such a figure doe- 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 interambulacrum 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 periMomal 

 and periproctal plates are easily injured and lost. It has been found a great help to dip tin- 

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

 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 .lipping, 

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

 ment; the spines will be firmly fixed in place, and the specimen is in much safer .-..u.li 

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

 lacral details, it has been found a great help to wet them with Ix-n/.ole. 

 sutures stand out clearly that are nearly or quite unrecogni/able in the dry state. 

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

 over night in a considerable volume of fresh water to remove the salt, then immer, 

 minutes in boiling water containing a liberal amount of corrosive sublimate m 

 treatment coagulates the albumen, poisons the specimens, and does no, affect 

 will dry out without odor, and in excellent condition as museum specimens, 

 the advantage of cheapness and quickness. When specimens are to be pr 

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

 removing much of the salt. 



