PLEUROBRANCHUS AGASSIZII MAC FARLAND 6l 



sal border is rounded, the ventral one is straight (PI. XII, fig. 

 56). Each mandible is made up of closely set chitinous elements, 

 arranged in some 72 transverse rows, each row containing about 

 35 platelets. The anterior portion of the platelets of each row 

 overlaps the interspaces between the posterior portions of those 

 in the preceding series, thus giving a close, tesselated appear- 

 ance to the whole mandibular plate. The individual elements of 

 the mandible are somewhat trapezoidal in form (PI. XI, figs. 44- 

 48). The anterior portion is prolonged above into a flattened 

 hook, directed obliquely forward and upward, pointed at the tip, 

 and bearing laterally three to six strong denticles. Immediately 

 behind the denticles upon each side is borne a stout truncated 

 lateral process, which is in contact with the corresponding 

 process of the adjacent plate of either side. The lateral pro- 

 cesses of a platelet are not, however, exactly opposite to each 

 other, the dorsal one being slightly behind its fellow of the oppo- 

 site side as a rule, thus causing a slight obliquity in the row of 

 platelets across the mandible (PI. XI, fig. 46). Toward the dorsal 

 margin the platelets become progressively thinner, until at the 

 margin itself they become flattened and scale like, the anterior 

 hook and the lateral processes are lost, and the whole takes on a 

 simple lozenge shape. The body of a typical platelet is thick, 

 truncate posteriorly, and fits closely in with its fellows. A deep, 

 narrow, median slit (PI. XI, figs. 45, 46) bifurcates the ventral 

 surface of the body of the platelet, extending backward nearly 

 or fully one-half of its length, and is slightly dilated posteriorly. 

 It does not extend through to the dorsal surface, but may be 

 readily seen from above by a slight change of focus. This groove 

 is well marked in all except the very youngest platelets, its loca- 

 tion in these latter being overlapped by a granular mass, identified 

 by Vayssiere as the remnants of the nucleus of the cell which 

 generated the platelet. 



The dimensions of these elements varies from the anterior 

 to the posterior ends of each mandible, and from end to end of 

 each transverse row. The length of a typical older platelet is 

 0.022 mm., its thickness 0.015 mm., and its width, inclusive of 

 the lateral processes, 0.017 mm. At the posterior end of the 

 mandible the length of a similarly situated platelet is 0.035 mm -> 

 and its width 0.019 mm. 



