158 SYNOPTIC COLLECTION. 



had a body composed of five basals and five radials. 

 This, however, is not the case with Trigonocrinus. Its 

 peculiar structure tends to prove that it is a form special- 

 ized by reduction. PL 282, figs. 1-5, illustrate the prob- 

 able evolution of this genus. Starting as a normal Crinoid 

 with five basals and five radials (fig. i) it loses in time 

 one basal and one radial and is like fig. 2. Three basals 

 then become larger at the expense of one, while two 

 radials increase in size (fig. 3). Trigonocrinus has 

 reached the stage represented by fig. 4, in which the three 

 basals are fused into one ring with only a vestige of the 

 fourth plate, while two of the radials are usually fused. 

 If this process of specialization by reduction is carried 

 still farther, the vestige of a fourth basal would disappear 

 and the two radial plates would become united, leaving 

 no suture, so that one could see only three basals and 

 three radials (fig. 5). 



An illustration of a Crinoid specialized by reduction is 

 found in Cheirocrinus (No. 283, model of C. r&rttrHaH). 

 For some reason the body with its drooping arms hung 

 downward from the top of the stem (No. 283). This 

 peculiar and unfavorable position has doubtless caused 

 the irregularity in the body plates, and a reduction in the 

 number of basals from five to three. The radials vary in 

 form and bear only three arms. 



The living Crinoids are represented in this Collection 

 by Metacrinus, Pentacrinus, Antedon, and Thauma- 

 tocrinus. 



The magnificent specimen of the living Crinoid, Meta- 

 crinus interruptus Carp. (No. 284), shows some of the parts 

 on a large scale. The long stem is nearly round at its 

 base, though it becomes pentagonal higher up. Many 

 jointed cirri are given off in whorls along the whole length 

 of the stem and the latter are closer together near the 

 body. The body itself is surprisingly small. It consists 

 of little basal and radial plates, while the lower plates of 

 the five arms help to make up a portion of its upper part. 



