BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. NIO 1. 23 



with that of tlie consumptioii of tliese, at least if it be as- 

 siiraed that the scale at tlie adoral top of the triangle, the 

 first detached, represents the solitary priinary iiiterradial of the 

 corona. 



The one of the binary columns of each interradiiim which 

 thns löses a peristomal plate, is that which about the same time 

 receives the increase of a new dorsal plate. Generally also the 

 columns corresponding in order all around are at the same stage 

 of growth. Thus, if in the a columns of all the five interradia 

 the new top-plate is more advanced than in the h columns, 

 tlie loss of substance at their adoral terminations is also grea- 

 ter; and, vice ver sa, if the h column exceeds at the top. it 

 löses inferiorly. From this rule, however, I found a few ex- 

 ceptions, and tliere may possibly exist some interchange in 

 this respect. Upon the whole, here as well as in the ambu- 

 lacra, since the number of plates in each column augments, 

 and their bulk goes on increasing, the dorsal growth of the 

 test must exceed the råte of reduction below. 



The reabsorption thus described as seen in the outward 

 aspect, is accompanied on the internal surface of the same 

 plates by a simultaneous process of another character, but of 

 a similar final efFect. 



When in a (Jidaris })apillata the peristomal circle is 

 viewed from the interiör of the test,' it is readily seen that the 

 three consecutive pairs of plates Avhich for the time being to- 

 gether constitute the adoral limit of the coronal interradium. 

 compared with the following plates are in a state of transfor- 

 mation. While they become depressed longitudinally they 

 grow high upwards at their adoral margins, the third plate 

 ■c, in a slight degree, the second, h, much more, and the first, 

 «, which at the time begins the column adorally, to such an 

 extent as to form a high and broad projection atFording ample 

 attachment to the whole of the external muscles of the corres- 

 ponding dental pyramid. In the astomous stage of the Goniocid- 

 aris, Pl. III, fig. 22, m. ijro, m. re, both pairs of these muscles werc 

 attached to the solitary primary interradial, near its adoral 

 margin. It is evident that when this plate was being reabsorbed, 

 they must have been received by the following binary plates 

 which afibrded extended attacliment on their growing projections, 

 and that, when these were melting away, they were transferred 



' See woodcut on next page. 



