IV. Trenton Limestone Edrioasters. 1914 162 



IV. The Edrioasters of the Trenton Limestone. 

 [Part II.] 



[Geol. Mag., n.s. , Dec. VI, Vol. I, pp. 162-171, Pis. XIII, XIV ; April, 1914.] 



TI1HE Cover-plates of the radial grooves are preserved here and there 

 X in specimen A, notably over the oral centre (PI. X, Figs. 1, 2, 5, 

 7, 8), but in B they are preserved over the whole of the grooves and 

 the month, though pressed down on to the floor of the grooves and 

 into the peristome (PI. XI, Fig. 1). Cover-plates are also present in 

 the British Museum specimens, and are almost complete in E 16054. 



Each cover-plate corresponds in position with a floor-plate ; so that, 

 like the floor-plates, the cover-plates form a double series of alternating 

 plates meeting in a zigzag median suture. The almost straight or 

 slightly rounded outer margin of each cover-plate fits into a bevelled 

 facet just within the rounded edge of the radial groove where the 

 floor-plate begins its downward slope (PL X, Figs. 5, 7). This feature 

 is well shown in specimen B, and there the floor-plate also shows, 

 on each side of the facet, and distinct from the peripodium, a slight 

 depression, possibly for the insertion of a muscle or ligament, or possibly 

 for the reception of the accessory cover-plate shortly to be described. 



From their facets the cover-plates stretch across the radial groove, 

 either in a straight line (PI. X, Fig. 5) or slightly arched upwards. 

 Often, however, as in B, they have been pressed down into the groove, 

 whether by contraction of tissues after death, or, as is more probable, 

 by the pressure of the superincumbent rock. 



The cover-plates abut closely, in tessellate fashion, when closed 

 in the normal position over the radial groove. When pressed inwards, 

 however, as in B, they have sometimes been made to imbricate with 

 adoral overlap. This suggests a possibility of the converse action, 

 namely, that in life they may have assumed a similar imbricate 

 arrangement when they opened outwards, thus forming a slight gap 

 between adjacent plates. 



A gap may also have been produced by the opening of the accessory 

 cover-plates. These are minute triangular plates frequently to be 

 observed, one at the outer end of each suture between adjacent cover- 

 plates, and therefore lying just over the pore (PI. X, Fig. 8). Minute 

 depressions in the floor-plates have already been mentioned as possibly 

 the facets for their reception. These accessory plates are not always 

 to be distinguished, but this may in some cases be due to the state of 

 preservation, and the difficulty of detecting the sutures bounding 

 such minute structures. When clearly seen, it is obvious that they 

 were definite elements demanding explanation, and not caused by 

 accidental cracks. 



On the other hand, there is in E 16054 an occasional appearance 

 of separate small plates along the median suture. In this case the 

 appearance is probably due to fracture. The specimen has been 



D 



