EXHIBIT AT PAN- AMERICAN EXPOSITION. 223 



THE DORSAL til'. 



(See Plate 63.) 



The dorsal cup in its simplest form is composed of two or three circlets of five 

 plates, those in one circlet alternating with the five in the adjacent circlet, Of these 

 the most important are those that support the brachia, and to them the term radiate 

 is restricted (colored blue). The interradial plates below these are the basals (col- 

 ored red), so called because in many crinoids they form the base of the cup and rest 

 on the stem. Such crinoids are called monocyclic. In other crinoids a circlet < if infra- 

 basals occurs beneath the basals (colored yellow), and these, therefore, are known 

 as dicyclic. 



Primarily there are five plates in each circlet; but owing to the fusion of two or 

 more of the proximal plates the number of basals in monocyclic forms may be reduced 

 to four, three, or even two, and the infrabasals in dicyclic crinoids to three. 



The cup often has, in addition to the plates above described, supplementary plates 

 known as "ancr/.s" and "interbrachiate," which assist in enlarging the cup. 



776. Batocrinus longirostris. Lower Carbonic. The cup plates not col- 

 ored are the interbrachials. 

 7519. Platycrinus planus. Lower Carbonic. 

 24191. Cyathocrinus multibrachiatus. Lower Carbonic. 



THE TEGMEN. 

 (See Plate 64.) 



The tegmen in its simplest form is composed of five plates called deltoids or orals 

 (colored red). There are nearly always present also ambulacrals (colored blue) 

 covering the food grooves or ambulacra that lead from the brachia to the mouth. 

 In many Paleozoic genera the ambulacra] plates are covered by the interradial dome 

 plates (colored yellow). When the dorsal cup is enlarged by other plates than those 

 of a simple crinoid, the tegmen also introduces supplementary plates known as intt r- 

 ambulacrals (black). Finally the tegmen may be in the form of a coriaceous skin, 

 in which large numbers of thin calcareous ossicles are embedded. 



The mouth is nearly always covered by the deltoids or the dome plates, while the 

 anus, which is closed by a valvular pyramid, is often surrounded by or raised on 

 small plates (colored brown). 



24,185. Platycrinus hemisphericus. Lower Carbonic 



768. Dorycrinus unicornis. Lower Carbonic. The anal pyramid is not. 



preserved. 

 775. Batocrinus elegans. Lower Carbonic. The anal tube is broken away. 

 24,413. Strotocrinus regalis. Lower Carl »onic. A natural cast of t lie interior 

 surface of the tegmen showing the ambulacra] grooves. 



THE BRACHIA AND PINM'LKS. 

 (See Plate 65.) 



The brachia or arms in their simplest form consist of a series of ossicles called 

 brachials (colored blue), which continue straight tip from the radiate (colored red). 

 The brachials may he in single or alternating double rows, and hence are spoken of 

 as uniserial or biserial. The inner surface of the brachia are grooved tor the trans- 

 mission of fooddaden water to the mouth and for the soft parts; and these are pro- 

 tected by covering plates called ambulacrate (yellow), which can open or close as 

 occasion demands. 



