24 THE MES0J501C KCHINODEHMATA OF UNITED STATES. [blll.W. 



shaped outline, the depression on the inner surface increasing until a 

 well-marked ambulacral furrow is produced. The plates are likewise 

 perforated a short distance in front of the same. The plates are united 

 in two ways, by articulation (PI. ii. Fig, ]e) and by sizygial suture (PI. ii, 

 Fig. Id), the former producing a movable joint, the latter nniting ad- 

 jacent plates immovably. Eows of pinnules are developed on the inner 

 surface of the arms alternately from opposite sides, each row branch- 

 ing from the epizygale, where the sizygial suture is observed (PI. ii. 

 Fig. Ic). 



The arms attain very gieat length. Grinnell mentions a specimen 

 examined by him where the arms measured 8 inches, and one examined 

 by the writer was very nearly as long. Grinnell thinks the arms may 

 reach 2 feet iji length. 



Related forms. — The present species is closely allied to Uintacrinns 

 uiestfalicus, described by Schliiter^ in 1878 from the Upper Cretaceous 

 (Senonian) of Eecklinghausen, in Westphalia. It is the only other spe- 

 cies of this genus thus far described, so that a statement of the affini- 

 ties of the two forms is important. In the conqiosition of the basis the 

 two species are very nearly identical. The radials and distichals show 

 very slight differences, but the brachials in U. socialis are decidedly 

 broader than in U. westfalicns. In the number and arrangement of the 

 iuterradials the most marked difl'erence is manifested. In U. socialis 

 seven interradial plates encircle the eightli, or eighth and ninth, as the 

 case may be, wbile in U. n-esifalicns the interradials are five in number, 

 all of which come in contact with plates of other areas. Several minor 

 differences appear in the rows of small plates that branch from the 

 distichals. Otherwise, how^ever, there is a marked similarity in tiie 

 arrangement and form of the several plates. In general outline- the 

 two species are very similar. 



Locality and geological horizon. — The first specimen was found on the 

 slopes of the Uinta Mountains, in Utah, by I*rof. Marsh, associated 

 with Ostrea congesta Con., a typical Cretaceous form. Later, others 

 were found in Kansas, associated with Odontornithes, Pterodactyls, 

 and mosasauroid reptiles, likewise ch.aracteristic of the Cretaceous. 

 Meek assigns the species to the Mobrara Group, a horizon of the upper 

 Cretaceous. 



Collections. — Peabody Museum, New Ilaven; U. S. National Museum. 



apiocri:n^ii)^. 



Calyx regular, composed of thick, articulated plates; basals five; 

 radials 5 x 1-3. Exceptions frequent. Arms strong, numerously 

 divided. Column lonff. 



> Zeitschr. der Deutsch. geol. Gesells., xxx, 1877, pp. 55-63, PI. iv. 



