38 SMITHSONIAN INSTITUTION 



lenses, separated by a thin partition, or diaphragm, which is pierced by the 

 pentapetalous or stellate canal. To form these lenses each columnal is usually 

 excavated on both faces (the ligament fossae), leaving an outer band of the 

 joint- face flat (the contact area), across which the radiating crenulse pass into 

 the depression toward the center; the apposed concave portions of the joint- 

 faces enclose the round lenticular chambers, resembling successive enlarge- 

 ments of the canal, which retains its smaller, pentagonal, or stellate form only 

 within the thin partition in the middle portion of the columnals (PI. V, figs. 

 2-5). In the upper part, where the columnals are extremely short, these cham- 

 bers are very wide, leaving only a narrow peripheral band of the full thickness, 

 and the canal through the partitions is also large; the partitions are very thin 

 in these parts, so that when the side of the stem is broken away for a consider- 

 able distance it looks as if there were a continuous canal filled with matrix, 

 almost the width of the stem (PI. V, figs. 2a, b, c). It is not improbable that 

 in these portions the partitions were so thin (perhaps membranous) that their 

 central diaphragms were destroyed during fossilization, or are indistinguish- 

 able from the matrix. Toward the distal end the concave excavated part 

 becomes gradually smaller, or may disappear, and the canal through the parti- 

 tions is more sharply stellate (PI. V, figs. 4a, b, 5). In some intermediate por- 

 tions the chambers are sometimes as narrow as they appear at the distal part; 

 and in some cases also the chambers appear relatively large in fragments 

 apparently distal. There is doubtless some variation in this respect, and in 

 some parts of the stems the chambers are absent in a number of consecutive 

 columnals; but the general rule is undoubtedly as above stated. These struc- 

 tures are fully shown by the series of figures on Plate V, above cited, made 

 from specimens ground, or naturally eroded, which will explain the facts 

 better than any description. Now it is evident that when the canal and its 

 enlargements have by infiltration been replaced by firm matrix, if a cross 

 fracture in the upper part occurs along the joint- face, the resulting section will 

 show the canal large and round; if through the middle of the columnal, the 

 canal will appear smaller, pentagonal, or stellate. Similar cross-sections toward 

 the distal part will show a round or sharply stellate canal, according to the 

 position of the fracture, the latter predominating. This will account for the 

 different varieties of stem sections and isolated stem joints seen in various 

 parts of the material. 



There is a frequent tendency in stems with a large axial opening and 

 branching or expanding root to become more and more modified toward the 

 distal end; and also for the columnals, after having increased in length for 

 most of the distance, to become shorter again at the end. This may be seen in 

 stems of Barycrinus, Anomalocrinus, and other forms. 



