56 THE CRmOIDEA CAMEEATA OF NOETH AMEEICA. 



basals to a less number, is readily understood among genera in which the 

 anal plate is wanting. When the base is quadripartite, it is invariably the 

 two anterior plates of the elementary five which are consolidated (Fig. 2). 

 In the tripartite base there is a fusion of the posterior with the left postero- 

 lateral basal J and another between the right posterior and adjoining antero- 

 lateral-plate (Fig. 3). The figure shows that a bisection of the two larger 

 plates will reproduce the original five pieces, interradially disposed. 



The case is not so simple in genera with an anal plate, where the form 

 of the basal disk is changed from pentagonal to hexagonal (Fig. 4), as a 

 bisection of the larger plates w^ould produce six plates instead of five. This 

 difficulty, however, is overcome if w^e consider that the introduction of the 

 anal plate into the ring of radials necessitated corresponding modifications 

 among the basals, as otherwise these plates would lose their interradial posi- 

 tion. It required either the introduction of a basi-anal plate, or an increase 

 in the size of the original pieces. That the latter occurred among the Came- 

 rata is clearly shown by the diagrams, and the evidence leaves no doubt at 

 what part of the base the extra width was inserted. 



Taking first the quadripartite base, and comparing Fig. 2 of the diagrams 

 with Fig. 8 — one pentangular and the other hexangular — w^e find that in 

 the latter the posterior basal has doubled in size (Fig. 7), without materi- 

 ally changing the orientation of the plates, or disturbing their general 

 arrangement. 



In the tripartite base the change w^as accomphshed in a different way. 

 There x is added to plate c (Figs. 9 and 10),- and the plates ah and ed have 

 coalesced, and hold relatively the same position as in Fig. 3. 



The bipartite base is probably derived from the tripartite (Fig. 4), which 

 preceded it in time, and x, which in the latter constituted a part of c, is 

 united with ed, and ah with c (Figs. 11 and 12). 



Now, taking up Fig. 7, and eliminating x, so that the side of plate a rests 

 against the plate e, we obtain Fig. 2, and by a similar procedure we are 

 enabled to transform Fig. 9 into Fig. 3. The hexagonal base is thus re- 

 stored to its primitive pentagonal form w^ithout disturbing the orientation 

 of any plate, compound or simple. 



A most beautiful confirmation of these observations is furnished by an 

 abnormal specimen of Teleiocriniis iimhrosiis in our collection, in which the 

 regular anal plate is w^anting. Teleiocriniis has normally three equal basals, 

 but in this specimen the basal plate to the left of the anterior ray is reduced 



