A MONOGRAPH OF THE EXISTING CRINOIDS 313 



and orals may be seen lying in two half circles which are open ventrally. The plates 

 of these two half circles do not correspond exactly to one another. There is some 

 shifting, as is also the case in Antedon. It is evident that the plates do not all appear 

 at one and the same time. Orals I and II are slightly larger, and therefore older, 

 than the other three orals, and basal V has just appeared as a minute grain, while 

 the other four have already begun to form small processes. The terminal stem plate 

 has appeared, together with five columnals. It is noticeable that the columnal 

 nearest the terminal stem plate is smaller than the next one, showing that it is not 

 the first formed. In another larva 24 hours old the skeleton is somewhat more 

 developed. The terminal stem plate and some of the basals and^orals are distinctly 

 branching. The columnals still number only five. Here, however, the one nearest 

 the terminal stem plate is the largest, so that in this case it would appear to have been 

 the first formed. Important new skeletal elements have appeared. At the upper 

 (oral) end of the column three very small calcareous grains are seen which are the 

 first traces of the infrabasals. Their definite number can not be stated from this 

 young stage, but the stages following afford proof that there are only three of them. 



In a larva 30 hours old the orals and basals, as well as the terminal stem plate, 

 have enlarged considerably, and are now more or less fenestrated plates. Their 

 relative position has changed somewhat so that they are no longer arranged in the 

 form of a horseshoe. The vestibulum could not be made out in this specimen, but 

 Mortensen says that judged from the curvature of the column the specimen must 

 be drawn in the same position as the first. Only two of the infrabasals are seen; 

 the third was probably quite concealed by this plate. The columnals have increased 

 considerably in breadth, assuming the shape of a half moon, their transformation 

 into the ring-shaped form taking place in the same way as ha Antedon. In this stage 

 some very small calcareous grains are seen between some of the normal columnals, 

 especially between the fifth and sixth and sixth and seventh. These perhaps may 

 represent new columnals interpolated between those first formed. However, it is 

 by no means certain that they are really intercalated columnals. On some of the 

 columnals small separate pieces are seen apparently soldered to the main piece of 

 the ossicle, and Mortensen was inclined to believe that this is the ultimate destiny 

 of all these small separate pieces, even the fairly large one lying between the sixth 

 and seventh columnals counting from the terminal plate. Seeliger found such inter- 

 calation to occur in Antedon, but only between the second to third upper (younger) 

 columnals, and only where the distance between the columnals is greater than usual. 

 Mortensen says this evidently means that there is merely a shifting in the time for 

 the first appearance of the columnals. 



In an embryo 40 hours old the skeletal plates have grown considerably, forming 

 large fenestrated plates. The infrabasals are fairly large fenestrated plates hidden 

 by the overlying basals, though seen quite distinctly on close observation. They 

 do not appear to be of different sizes. The number of columnals appears to be ten. 



In a normal embryo shortly after fixation the oral and basal plates have assumed 

 their final position, forming the calyx, and the columnals have begun to increase in 

 length. There are ten of these, the uppermost mostly hidden by the basals. The 

 infrabasals can not now be seen because of the opaqueness of the embryo. The 



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