PLATE :,. 



Strongylocentrotus drobachiensis (O F. Muller). 

 Figs. 1-12. Mt. Desert, Maine. Studies of the lantern and muscles of the same. Page 178. 



Fig. 1. Lantern with muscles and perignathic girdle. X 4. Dental capsules, cap., very large; intercompass muscles, i, 

 pass beneath the dental capsules; protractor, pr., and radial compass muscles, r, are inserted on low apophyses; 

 retractor muscles, re., are inserted on auricles. (Compare text-fig. 229, p. 193.) 



Fig. 2. The lantern, facing pyramid 5, showing interpyramidal muscles, styloid processes of the dental slides, epiphyses, 

 crests, and compasses. X 4. (Compare text-fig. 213, p. 184.) 



Fig. 3. Pyramid of interradius 4 in side view, with corrugations for the attachment of interpyramidal muscles, epiphysis 

 with its glenoid cavity, crest, external and internal tubercles. X 4. 



Fig. 4. A pyramid, showing the origin of the protractor and retractor muscles on the pyramid and epiphysis, left epiphysi.s 

 removed. The pyramidal suture, p. s.; styloid processes, s(. of the dental slides are visible above the base of the 

 deep foramen. X 4. 



Fig. 5. Pyramid of interradius 4 seen from the center, showing dental slides, corrugations for muscle attachment, and the 

 epiphyses with their glenoid cavities, external and internal tubercles and crests. X 4. 



Fig. 6. Left half-pyramid, the right half being removed, showing tooth and dental capsule in place; also, epiphysis, ep.; 

 brace, b; and intercompass muscle, i; the pyramidal sutural face, p. s., is shaded. X 4. 



Fig. 7. Upper part of pyramid, with dental capsule in place: styloid processes are prominent. X 4. 



Fig. 8. Tooth flattened out. X 4. At the basal growing portion, which lies in the dental capsule, the tooth is grooved, 

 as is the whole tooth in the Cidaridae and other primitive types. Lower down a keel develops, as shown by sections 

 at the points indicated. 



Fig. 9. Top of lantern to show the structure. X 4. At areas 2, 3, a whole tooth is in place; at 1, the pulpy part of (he- 

 tooth is removed; at 4 and 5 the tooth is-removed. At area III the compass, brace, and epiphyses are in place; at V, 

 the compass is removed; at IV the brace also is removed to show the epiphyses; and at I the epiphysis of one side i.s 

 removed to show the pits in the top of the pyramid. 



Fig. 10. A pyramid of interradius 5 seen from above. X 4. The epiphyses are removed to show the top of the pyramid 

 with its characteristic pits. 



Fig. 11. A brace, seen from below. X 4. 



Fig. 12. Epiphyses with brace in place to show internal and external brace muscles, c. b., i. b. X 5. 



Fig. 13. Massachusetts Bay. Diam. 52 mm. R. T. J. Coll., 657. X 3. Madreporic pores are in genitals 2 and 3; 

 oculars I, V insert. Page 172. 



Fig. 14. York, Maine. Diam. 46 mm. R. T. J. Coll., 779. X 3. Madreporic pores are in genitals 1, 2, 3, and oculars 

 II and III. Page 173. 



Fig. 15. York Harbor, Maine. Diam. 47 mm. X 3.6. Oculars I, V insert; an interspace exists between genital 5 and 

 ocular I where the periproct reaches interambulacrum 5 as in some Echinothuriidae. a very rare variant. (Compare 

 text-fig. 170, p. 149.) Pages 63, 177. 



Fig. 16. Calderwood Island, Fox Island Thoroughfare, Maine. Diam. 36 mm. R. T. J. Coll., 822. X 3.7. Partially 

 tetramerous; interambulacrum 1 is wanting, so that ambulacra I, II are in contact throughout. There are five 

 oculars, V only insert, and five genitals, but genital 1 is impcrforate and lies dorsal to oculars I. II, a very rare varia- 

 tion. (Compare Plate 1, fig. 2.) Pages 42, 167. 



Lettering: 6, brace; c, compass; cr., crests of the epiphyses; e. b., and i. b., external and internal brace muscles. 

 extending from the braces to the epiphyses; ep., epiphyses; gl., glenoid cavity; i, intercompass muscles; ip., inter- 

 pyramidal muscles; I. p., left half -pyramid; p, pyramid; pr., protractor muscles; p. s., pyramidal suture; r, radial 

 compass muscles; re., retractor muscles; r. p., right half-pyramid; si., styloid process of the dental slide; /, tooth; 

 . tu. e. and tu. i., external and internal tubercles by which the epiphyses interlock with the brace. 



