152 ROBERT TRACY JACKSON ON 



Strong YLOCENTROTis drobachiexsis. 



Fig. 112. Intel-ambulacrum of a very young specimen (measuring 1.2 mm. in iliameler), showing plates of interambu- 

 lacrum from ventral side, the single ventral plate 1, and succeeding plates 2, 8, have each one .spine boss. (Loven, -92.) 

 X about 55. P. 129. 



rig. 113. Interambulacruui of a little older .specimen (measuring 2.2 mm. in diameter) showing a partial resorption of 

 single ventral plate 1, and two spine bosses on the second plate of the right hand column. (Lovgn, '92.) X about 32. P. 129. 



Fig. 114. Interambulacrum of a young specimen (measuring 7 mm. in diameter) showing 11 plates iu interambulacral 

 columns. Compare plate 11 with all plates of Fig. 112, plates 11 and 10 of Fig. 115 and plates 11 and 21 of Fig. 116. X6. 

 P. 129. 



Fig. 115. Interambulacrum of an older specimen (measuring 25 mm. in diameter), showing 10 plates iu interambulacral 

 cohunns. The youngest plate 16 of the left-hand column, resembles all the plates of Fig. 112, also plate 11 of Fig. 114 and 

 plates 21-22 of Fig. 110. Compare also with plate 16 of Fig. 116. X6. P. 129. 



Fig. 116. Interambulacrum of an adult specimen (measuring .52 mm. in diameter) showing 22 plates ui interambulacral 

 columns. The youngest plates, 21 and 22, have a single spine boss and are comparable to all the plates of Fig. 112, plate 11 

 of Fig. 114, and plate 16 of Fig. 115. X3. P. 129. 



Fig. 116a. The fifteenth plate of the left hand column of Fig. 110, enlarged to show full detail of surface ornamentation. 

 X6. Figs. 114-116, Mas.sachusetts Bay. P. 130. 



Figs. 106-1 16 drawn by Mr. J. H. Emerton. 



PLATE 25. 

 Pl.vcenticeras placenta. 



Fig. 117. The immature portion of a large specimen. The jijies 118, 119, 120. 121 show the po.sitions of the septal 

 sutures represented in Figs. 118-121. xl. Cretaceous, Dakota. Coll. Prof. A. Hyatt. P. 1.34. 



Fig. 118. A comparatively early septum, all saddles and lobes entire. Enlarged. P. 134. 



Fig. 119. A later septum more complicated than Fig. 118. Lobes and saddles near the umbilicus are shiiilar to all the 

 lobes and saddles of Fig. 118. Enlarged. P. 135. 



Fig. 120. A later septum still more complicated. The saddles and lobes near the umbilicus are like all the saddles and 

 lobes of Fig. 118. For other comparisons, see text. Enlarged. P. 135. 



Fig. 121. A later septum more complicated than Fig. 120. The lobes and saddles close to the umbilicus are simple 

 like all the lobes and .saddles of Fig. 118. For other comparisons, sec text. Enlarged. P. 135. 



Cladiscites tornatus. 



Fig. 122. Showing septal suture simple toward umbilicus, complicated on ventral portion. Triassio, Hallstadt. (Zittel, 

 '76-'90.) P. 137. 



Piiyli.oceras nilssoni. 



Fig. 123. Showing suture line simple toward umbilicus and progressively more complicated toward ventral border. 

 Jurassic. (Zittel, '70-'flO.) P. 137. 



Metacrinus rotundus. 



Fig. 124. Showing dense association of cirrhi and pentagonal form of stem near the calyx. Fig. a, section of same 

 (drawn from surface study). XU. P. 127. 



Fig. 124a. The same specimen, portion of stem at a considerable distance from calyx, showing widely spaced cirrhi 

 and nearly round form of stem. At the base of figure the plates of the stem are indicated. Fig. a, section of stem (drawn 

 from surface study), xii. Fig. 124, Japan, Coll. Bost. soo. uat. hist. P. 127. 



