DESCRIPTION OF PLATES. 



In the plates, the numbers designating figures are in heavy-faced italics, and in all cases they occur below the figures, 

 usually on the left lower side. In order to see the full detail of the heliotype photographic plates, also Plate 57, they should 

 be examined with a reading glass. In most cases only a single page reference is given, which indicates the page on which 

 the species is described. It is inexpedient usually to give references to the morphological or other parts of this memoir 

 where the figures are discussed. 



When the correct axes of a specimen are known, these are indicated by I lie Loveu method of enumeration, the ambulacra 

 being numbered I, II. III. IV. and V, and the interambulacra 1, 2, 3, 4, and 5, the numbers revolving from left to right, or 

 clockwise; when the specimen is viewed from below, Plate 2, fig. 1; when viewed from above, the numbers revolve counter- 

 clockwise, Plate 2, fig 3 



When the correct axes of a specimen are not known, as in most Palaeozoic Echini, the areas are lettered arbitrarily A, 

 B, C. D, E, F, (!. II. I, and .1, the ambulacra being lettered B, D. F, H, and .1. The lettering revolves from left to right 

 when the specimen is viewed from above, and in external view, Plate 11. fig. 2; when viewed from below, the numbers revolve 

 counter-clockwise, Plate 11. fig. 1 (p. 20). 



It is important to bear in mind that when a specimen is viewed from the interior (an external mold is necessarily in 

 internal view), the orientation and the lettering of ambulacral and interambulacral columns of plates are reversed from thai 

 -ecu from the exterior (p. 20), Plate 23, figs. 1 and 3 (compare Plate 55, figs. 1 and 2). 



In the test as a whole, the lettering is as follows: a. b, the left and right columns of ambulacral plates in an area, Plate 

 31, fig. 1; or a, a'; b'. b. the two columns of demi-plates, o, b, and the two columns of occluded plates, a', b', when then- are 

 four or more columns of ambulacral plates in an area, Plate 42. tig. 2; Plate "ill, lig. 4. 



The interambulacral columns in an area are numbered from 1 upward as they appear passing aborally in a given area, 

 Plate 26; Plate 4.). tig. 1. II. the heptagonal plate, associated with the initial pentagonal plate of a new column as intro- 

 duced, is seen in many Palaeozoic Echini. Plate 4">. fig. 1; g, genital plate; <«-., ocular plate; and sr., suranal plate, Plate 4, 

 figs. 1, 2, and 7. The letter X has no definite meaning, hut is used to call attention to specific parts, Plate 25, figs. 1 and 2. 



Lettering of the parts of tin- Aristotle's lantern with the associated muscles i.-, as follows; /,. brace; c, compass; cap . 

 dental capsules; can., condyles of brace; cr., crests of the epiphyses, when such an 1 developed; i . b., external brace muscl • 

 i p., epiphyses; fo. < ., and fo. i., external and internal foramina in the brace for articulation with the tubercles of the epiphyses; 

 gl , glenoid cavity of an epiphysis, which receive- the condyle of the lira,': i. intercompass muscles; i. b., internal brace 

 muscles; ip., interpyramidal muscles; I. p. kit half of a pyramid: pr., protractor muscles of the lantern; p. s., pyramidal 

 suture that divides the left and right half of a single pyramid; r, radial compass muscles; re., retractor muscles of the lantern : 

 r. p., right half of a pyramid; s, suture, applied to different parts: s. </.. dental slide that supports the tooth; St., styloid 

 processes which are the dorsal extension of the dental slides in certain Echini: (. tooth; tu. e., anil lu. /.. external and internal 

 tubercles of the epiphyses which interlock with corresponding foramina of the braces, Plate 2, figs. 7-17. and 19-21; Plate 5, 

 figs. 1-12. 



