210 PROCEEDINGS OF THE ACADEMY OF [1881. 



spirally on tlio outside, while the other end is directed toward the 

 oral centre, but a connection with the food grooves has not yet 

 been observed. 



In all cases where the oesophageal network has been examined, 

 among the Actinocrinidai and Platycrinidse, it varies only in out- 

 line and in the number of whorls ; while in Ollacrinus the entire 

 skeleton resembles a large spiral. In the only specimen observed, 

 it apparently consisted of a large round canal, which turned 

 spirally on its axis, and which near the basal plates turned 

 upward, but the organ is in a fragmentary condition, and it is 

 ((uite possible that this part was surrounded by other convolu- 

 tions. Until recently this organ had been observed only in 

 crinoids from the Burlington group. Angelin, however, in the 

 Icononographia Crinoideorum Suecise, figures several examples 

 from the Upper Silurian of Sweden ^Pl. 26, figs. 12, 12 a, 6, ). It 

 is well preserved in these specimens, and resembles that of later 

 Actinocrinida\ It differs, however, in being closed at the outer 

 side, while the inner parts, as in Burlington specimens, are 

 distinctly coiled. The outer wall is pentangular in outline, open 

 toward the basal disk, and consists of a very delicate porous 

 texture, appearing like an envelope for the inner or coiled parts, 

 and as such possibly represents the perivisceral plexus, which in 

 some cases almost equals the oesophageal network in delicacy of 

 structure. 



8. Column. 



The column in the SphseroidocrinidiB is generally circular 

 transversely, though sometimes elliptical or pentagonal and even 

 quadrangular. It is elliptical only in Platycrinua^ and the 

 pentagonal form occurs only in Befeocrinus and some few 

 species of the Glyptocrinites. The central canal varies from large 

 to extremely small, and is round or pentagonal. In Platycrimis 

 it is so minute that in columns of an inch or more in thickness 

 on their long diameters, the opening will scarcely admit the point 

 of a needle. In the Rhodocrinidse it is irregularly^ pentagonal, 

 and as a rule small. Among the Actinocrinidte also, the passage 

 is generally not aboA'e medium size, but in Megistocrinus it is 

 remarkably large throughout the column and all its branches. 



B}' the earlier writers, new species were often based upon frag- 

 mentary columns, a practice which ha^ fortunately been aban- 



