494 CLARK: CRINOID FAMILY PLICATOCRINIDAE 



the changes in its excess energy content. In calcite the change 

 in crystal angles with temperature is practically linear up to 600°. 

 This indicates that in quartz the excess in energy content repre- 

 sents an internal reaction or equilibrium condition which even- 

 tually causes the inversion. It will be of interest to study other 

 minerals, showing inversion points, with respect to their energy 

 content and crystallographical and optical properties at different 

 temperatures; also to ascertain the form of the curves near the 

 melting points. 



PALAEONTOLOGY. — The systematic position of the Crinoid 

 family Plicatocrinidae. Austin H. Clark, National Museum. 



In the preparation of the section dealing with the Crinoidea in 

 the new edition (1913) of Eastman's translation of Zittel's Palaeon- 

 tologie, Springer and Clark^ were unable to come to a satisfactory 

 conclusion regarding the proper systematic position of the family 

 Plicatocrinidae (as defined by us including the genera Plicato- 

 crinus, Hyocrinus, Gephyrocrinus, Thalassocrinus, Ptilocriiius and 

 Calamocrinus) , which we tentatively placed among the Crinoidea 

 Articulata, just beyond the comatulids, the young of which the 

 species in this family greatly resemble. 



The Articulata include all of the other recent crinoids so far 

 discovered and this fact, together with the close resemblance of 

 the species of Plicatocrinidae to the young of the comatuHds, was 

 largely responsible for our placing this family here. But the 

 species of the Plicatocrinidae differ from all of the other forms in- 

 cluded in the Articulata in a number of most important and fun- 

 damental features. 



1. The tegmen is entirely covered with a pavement of solid 

 plates. 



2. The orals are always present; they are not plane or spherical 

 triangles, as they are in all of the (comparatively few) species of 

 Articulata which possess them, but their edges are turned upwards 

 so that instead of presenting five sharp angles to the mouth they 



^ When this paper was prepared and sent to press Mr. Springer was absent from 

 Washington, but he returned to the city just before its publication. After a dis- 

 cussion of the subject matter he requested me to add a note stating that he is in 

 complete accord with my conclusions as herein expressed. 



