104 CLEIOCEIXUS. 



corners of the primibrachs, and toward the right, the corner of the right 

 posterior radial, which differs slightly in shape from the other radials. In 

 this figure the relative thinness of the plates of the outer circlet can be 

 readily seen. Fig. 5 b is a more nearly side view of the same structure, 

 enlarged four diameters. Here the angular lower edge of the radial of 

 the outer circlet can be well seen, and the drawing shows very clearly 

 the way in which these plates hang suspended to the outside of the 

 inner ones, and project downward over the column without anything for 

 their angles to meet or rest upon. In both the last figures there appears a 

 depression at the middle of the upper edge of some of the inner plates. 

 This is actually a shallow, lip-like groove, plainly visible on three of the 

 plates, apparently traversing the inner surface of these plates in a radial 

 direction. I cannot trace it upon the surface of the succeeding primibrachs 

 which are preserved, but the inner surface of these plates is somewhat ob- 

 scure, and did not come out freely in cleaning. If this groove be taken as 

 showing the position of the axial nerve cord leading into the column, it 

 would seem to indicate that the axial canal occupies morphologically its 

 proper radial position in the calyx, notwithstanding its interradial cross- 

 section below the calyx. Fig. 6 is an attempt to show by a diagram of a 

 vertical cross-section, the relative position of the plates of the inner and 

 outer circlets, and of the primibrachs succeeding them, and also how the 

 outer plates project downward over the column. 



These illustrations will give a good idea of the structure as I interpret 

 it from the most careful study of the specimen which it was possible to 

 make. It will be seen from them how large and strong these inner plates 

 are. They form a rigid ring surmounting the column, of much stronger 

 construction than the succeeding calyx plates ; and as a result of this the 

 base, reinforced by the outer circlet also, usually retains its shape, while the 

 calyx above is greatly flattened by pressure. 



Now this sort of a base is absolutely anomalous, and without precedent 

 anywhere. The five inner plates resting upon the column, being radial in 

 position, must be taken to be the infrabasals. But what has become of the 

 basals ? It is plain to me that they are the so-called interradials, which, by 

 some strange freak of nature, have morphologically — not accidentally- 

 become displaced from their proper position in the calyx wall, being pushed 

 in between the radials, and both having slipped down over the infrabasals. 

 Or, supposing a reverse process to have taken place, we may consider that 



