t'LATYCUIN ID-E. rLATYCRINITES. 13 



the same appearance as the cellular tissue of plants when the stem is cut across. This 

 mode of structure would render a large central columnar canal less necessary, and 

 accordingly we find in the Plati/criiiites, and some other genera, only a minute perfora- 

 tion occui3ying the centre of the column. This circumstance, at one period, embarrassed 

 the scientific inquirer, who was unable to discover the manner in which the fimctions 

 could be carried on by means of so minute an apertiu-e. But the microscope has 

 revealed to us in the Pentacrmites a most beautiful and distinct tubular structui-e, and 

 wliich we beUe^e to be analogous in the Platt/crinites. 



AVe are now engaged in a series of investigations regarding the various columns, and 

 on a fiitiu'e occasion hope to present a yet more satisfactory elucidation of the svibject. 

 The merit of discovery, as to the internal structure of the column of Peiitacrinifes, is, we 

 believe, justly due to Dr. Carpenter, who, after much care and trouble, was enabled to 

 produce some liighly interesting and instructive specimens, distinctly sho-vving the 

 tubular arrangement in the column. 



The muscrdar substance which occupied the columnar tubes probably ramified into 

 exceedingly thin transverse fibrous cushions, which were interposed between the 

 columnar joints, adding, by their contractile power, to the flexure of the column. 



Miller asserts that the column at its attachment to the body is "of an almost round 

 figure, and that the transverse ridge is here very faintly exhibited." Neither of these 

 assertions is correct, for the column is perfectly circular at, and for some distance below 

 its junction with the dorso-central plate ; and there is not the slightest indication what- 

 ever of the transverse ridge near the point of adherence, as many jomts with well 

 defined diverging stria; occur before the central ridge becomes visible on the facets of 

 the joints. Miller also considered the attachment between the column and body to have 

 been slight in this species. There is, however, no evidence to warrant this opinion, for 

 the union between the column and dorso-central plate is, to all apjiearance, as well 

 secured, and its adlierence as perfect as in others of the same family. 



Auxiliary side Arms. — Distant about a thu'd from the base of the column auxiliary 

 side arms occur. These arms are long, flexible, and equal jointed, with a central per- 

 foration communicating Avith the column ; they are generally situated at the ends of the 

 longest diameters of the eUiptical columnar joints. From this form of structure it 

 follows as a necessary consequence, that when the eUiptical joints assume an oblique 

 direction to each other, the side arms are brought round in various directions, according 

 to the degree of obliquity which each separate joint assumes in respect to those adjoin- 

 ing it, so as to present a radiated figure somewhat resembling the radii of a coach wheel. 



The joints composing the auxiliary side arms articulate by a radiating striae, as in the 

 circular portion of the cokunn. 



We ha-^e akeady observed that the form of the column would probably fiu-nish a clue 



