ON SOME PELMATOZOA FROM CHAZY LIMESTONE OF NEW YORK WJ 



trated the theca to the depth figured by Billings and so deep a pene- 

 tration in his specimen may have been the result of partial crushing 

 or deformation due to pressure. 



Taxonomic position. Before attempting to discuss the position 

 of this curious species with its approximately 50,000 plates and 

 ossicles it may be well to point out that the close to 90 plates of its 

 aboral surface do not necessarily point to a generalized type of low 

 rank. 



A period of stress developed the many centers of stereom forma- 

 tion and the numerous and irregular plates of a form like Eocystis, 

 but protection in this direction once secured in its adjustment to its 

 environment, there could occur the passive loss, through the 

 mechanism of inheritance, of a plate or so at a time and the others 

 would simply extend their surfaces a little more to keep up a com- 

 pact exterior. New crossings would tend to replace loss, but a 

 Alendelian factor has entered that tends to simplicity and though 

 loss be slow it is nevertheless sure until it begins to interfere with 

 some other function. Thus the few thecal plates of a Cryptocrinus 

 are indicative of a higher genetic position. A period of stress for 

 some other function would require response or extinction and again 

 lead to proliferation of parts as in stem or brachiole development. 

 The law briefly expressed is that the quiet of an unexacting environ- 

 ment for any part leads to numerical or other simplicity of that 

 part, and that the stress of an exacting environment, on still plastic 

 parts, leads to gain in numerical or other complexity of that part. 



Our form seems to have been living in a period of stress in rela- 

 tion to respiration and reproduction. We therefore find five points 

 of what we may call primary meristem, developing adambulacrals, 

 covering plates, wing plates, brachioles, plates lining brachiolar 

 chambers, and hydrospires ; and exciting the neighboring deltoids 

 and bibrachials to constantly add to their area. This increase of 

 area to the strong bibrachials would tend to lift the oral surface 

 away from the interbrachials. These interbrachials are away from 

 the points of activity and any extra activity on their part might lead 

 to serious interference with the larger water exits of the deltoids. 

 Release of pressure would render the water outflow the easier and 

 in the membranous margins of the extending ends of the hydro- 

 spires new centers of stereom formation would naturally arise and 

 fill in a series of plates representative of the external stereom thick- 

 ening of the hydrospire folds of Codaster. The result would be 

 a partial fourth circlet of supplementary plates, either homolo- 

 gous in part with the stereom formation of the inner surface of the 



