CRETACEOUS ECHINODEBMATA. 37 



mens found in place and unweathered. He says that the long arms were so entangled as to make 

 it very difficult to trace them to their extremity. 



The collection by H. T. Martin of a large number of specimens of Uintacrinus socialis from 

 Williston's locality in western Kansas furnished several museums, among them the British 

 Museum and the Peabody Museum of Yale University, with new and valuable material. The 

 material acquired by the British Museum was very thoroughly studied in 1895 by F. A. Bather, 

 whose morphologic study of this species added much to what was previously known. A large 

 slab acquired by Yale University was described by C. E. Beecher in 1900. 



The complete description of Uintacrinus socialis and the true interpretation of its structure 

 and relations were not made until 1901, when Springer obtained from western Kansas, through 

 the intelligent collecting of H.'T. Martin, a large and finely preserved lot of specimens, far more 

 than had come into the possession of any of his predecessors. With his wide and accurate knowl- 

 edge of the crinoids he has been able to add more than any of his predecessors to a knowledge of 

 this interesting form and in addition to the discovery of clicyclic as well as monocyclic individuals, 

 was also able to find a number of specimens with well-preserved tegmens so that for the first 

 time a knowledge of the structure of the ventral surface of Uintacrinus was secured. The fol- 

 lowing description is largely based on Springer's elaborate discussion of this form. 



The calyx is globose with 10 long simple arms bearing pinnules. The test must have been 

 flexible. The calyx plates are thin, frequently convex and joined together by loose sutures, the 

 faces of which are at times slightly grooved. The calyx is without column or stalk and belongs 

 to the class of free forms. 



The basis is composed of a centrale, surrounded by either a circlet of basals or a circlet of 

 infrabasals, which are in turn surrounded by the basals. The centrale in the first form has its 

 angles radially directed and in the second interradially directed. The form is, however, proba- 

 bly dicyclic. The infrabasal circlet is not complete in all specimens, some having only 4, 3, 2] or 

 even 1 infrabasals. 



The radials are five in number and surround the basals, with which they alternate. They are 

 equal, six or seven -sided, the latter occurring where the edge toward the basis forms a reentrant 

 angle. The radial plates are wider than high and come in contact with one another oidy in the 

 lower portions of each side. They come in contact with the proximal interbrachials and support 

 the first primibrachs. 



Following the radials are the primibrachs, secundibrachs or distichals, and fixed pinnules, 

 all of which enter into the formation of the calyx, to which are added interbrachials, interdis- 

 tichals, and interpinnulars. It is not always easy to determine the exact limits of marginal- 

 attachment of the fixed brachial elements in the fossil state. There are two primibrachs, the 

 first (IBrl) hexagonal in form abutting on the radial on its lower margin, and the second primi- 

 brach (IBr2) on its upper margin where latterly it comes into contact with interbrachials on either 

 side. Both the first and second primibrachs are somewhat narrower than the radials. The 

 second primibrach is axillary and generally pentagonal in form. It also abuts on interbra- 

 chials on either side and bears upon its upper margin the first of the secundibrachs. 



The secundibrachs or distichals are commonly fixed in the first eight plates with the aid of 

 the fixed pinnules, interbrachials, interdistichals, and interpinnulars. Those secundibrachs 

 bearing fixed pinnules appear slightly axillary, which gives the appearance of a somewhat irregu- 

 larly depressed series. 



The normal order of succession of the fixed pinnules is first from the second secundibrach on 

 the outer side, the next from the fourth secundibrach on the imier side, then from the fifth secun- 

 dibrach on the outer side, then the seventh secundibrach on the inner side, and finalty from the 

 eighth secundibrach on the outer side. The intervening secundibrachs do not bear pinnules. 

 Beyond this point each brachial bears generally on alternate sides a pinnule except where there 

 is a syzygy when only the epizygal bears a pinnule. The fixed pinnules comprise four or five to 

 ten of the proximal plates. The number of plates is greater in adults than in the young forms. 

 United with the interbrachial and in many specimens with the interphmular plates they form 

 the interbrachial area of the calyx, and with the intersecundibrachs or interdistichals the inter- 



