38 MESOZOIC AND CENOZOIC ECHINODEBMATA OF THE UNITED STATES. 



distichal area of the calyx. Each interbrachial area contains in adults, usually six rarely eight, 

 of these fixed pinnules. In the young four and even two have been found. The interdistichal 

 area contains four fixed pinnules in adult forms and two, and at times apparently none, in the 

 young. The amount of fixation therefore increases with age. 



The interbrachial area consists of from three or four to twenty- three plates. The arrange- 

 ment of the plates is very variable. When there are seven plates or less no one of the inter- 

 brachials is inclosed, but the plates are disposed in more or less irregular vertical rows. This 

 arrangement holds in some specimens containing eight or even nine or ten plates. In other 

 specimens one or more plates are inclosed. Variation in the interbrachial areas occurs in some 

 individuals.. 



The intersecundibrachs or interdistichals may differ in the same way, the number varying 

 from none to 8, although the most common number is from 2 to 5, the younger forms having less 

 than the adults. The interpinnular plates are not present in all specimens, but one or two may 

 be placed between the proximal portions of the fixed pinnules and the intervening secundibrachs. 

 The interpinnulars generally appear in corresponding position on opposite sides of the same area. 



The arms are the continuation of the secundibrachs or distichals and are therefore ten in 

 number. They consist of brachials and pinnules, the latter disposed in the manner previously 

 described. An arm branch 100 centimeters, or 40 inches, long has been described by Springer, 

 who thinks it not improbable that a total spread of arms of 250 centimeters, or 8 feet 4 inches, 

 may have occurred. 



The ventral disk or tegmen has been described only by Springer, who prepared a number of 

 specimens on which the disk was beautifully preserved. He states that it is composed of the 

 plated skin, the membrane being of such a highly carbonaceous composition that it is jet black 

 in the fossil. This membrane evidently inclosed the entire visceral mass and formed a lining 

 where it is usually seen when the calyx is broken away. Upon the disk it is studded or paved 

 with small calcareous plates or spicules which are not connected by suture but are embedded in 

 the tegment touching each other. They are of irregular shape, variable in size, and without any 

 definite plan of arrangement. 



The central part of the disk is occupied by a large, conical anal tube, shaped like an inverted 

 funnel, which is perhaps an extension of the ventral disk above described. It is also composed 

 of a plated skin, the granules of the disks passing gradually into it and becoming more and more 

 elongate, until toward the distal end where the opening was they become threadlike. 



The mouth is excentric, and the ambulacra diverge at a point near the margin of the disk. 

 Two of them follow around the margin in a large hood-shaped cone, closing the anal tube. 

 These branch on either side so as to connect with the arms of the posterior rays. The next pair 

 of ambulacra are shorter; they branch and supply two lateral rays. A single groove runs to 

 one branch of the anterior ray and the other branch does not seem to have any groove leading to 

 it. The ambulacral grooves are simply depressions in the peristome. There is no trace of any 

 covering plates or anything like an ambulacral skeleton. 



Localities. — The first specimen of this species was found by O. C. Marsh in the Uinta Moun- 

 tains, Utah. Grinnell established the genus Uintacrinus on much better preserved material 

 from Trigo County, Kans. Still better specimens were later found in Logan County, Kans., which 

 has been the source of most of the fine material in the various museums of the world. It is from 

 this locality that Springer secured a few years ago the superb specimens that are the basis of 

 his elaborate monograph. 



Geologic Tiorizon. — Niobrara chalk, Upper Cretaceous. 



Collections. — Museum of Comparative Zoology, Cambridge; U. S. National Museum (S044); 

 University of Kansas; Yale University; British Museum; Johns Hopkins University. 



