46 The Ohio Journal of Science [Vol. XXI, No. 2, 



Casts of the interior of the theca. — At that end of the cast which 

 corresponds to Figure 11 on Plate 12 accompanying Hall's original 

 description of Lysocystites nodosus, three short narrow radiating ridges 

 occur, and the outHnes of three basal plates may be detected. Two of 

 these plates are truncated above and the third is more rhomboid in 

 outline and is angular at the top, as in the basal series of plates in Aetho- 

 cystites. Along the upper margin of this -basal series of plates are five 

 more or less cuneate nodes, equidistant from each other, corresponding 

 in position to the lower part of the sutures between the plates belonging 

 to the second series. These cuneate nodes on the cast correspond in 

 position to the lower series of strong angular nodes on the exterior of 

 the theca. In addition to the five cuneate nodes there are three narrow 

 vertical ridges, usually boldly defined, located at the upper end of the 

 sutures between the three basal plates, and extending thence upward 

 into the median part of those three plates of the second series which 

 are angular at the base. All of these structures on the cast of the interior 

 of Lysocystites correspond to salient features on the exterior of Aetho- 

 cystites. Orientation of the specimen is facilitated by the fact that that 

 one of the three short radiating ridges which is directed toward one of 

 the nodes belonging to the lower series follows the median line of the 

 narrow rhomboid basal plate. Passing from the node at the top of this 

 plate two nodes toward the left, and thence vertically upward, the anal 

 aperture will be found at the top of the third series of plates, slightly 

 toward the right of this vertically directed line. 



In Hall's Figure 11, the base of the theca has been interpreted as the 

 summit. The specimen is so oriented as to place the rhomboid basal 

 plate at the top of the figure. The narrow vertical ridge at the lower end 

 of the figure was determined incorrectly as the ovarian aperture; no 

 aperture of any kind being present here. The two other corresponding 

 vertical ridges, at the upper end of the sutures on the right and left of 

 the rhomboid plate are not clearly defined in the cast figured by Hall, 

 and are not indicated in his figure. Figure 10 represents the type in an 

 inverted position, with the supposed ovarian aperture occupying the 

 iniddle of the upper part of the figure. 



Returning to the Walker Museum specimen described above, a 

 second series of cuneate nodes is located immediately above the first 

 series, at the top of the second series of plates, but with the pointed end 

 directed in the opposite direction. They locate the upper series of nodes 

 visible on casts of the exterior of Aethocystites. The truncated ends of 

 both the lower and upper series of cuneate nodes are separated more 

 or less by a transverse groove from the rest of the nodes. If any pores 

 connecting the plates are present they might be searched for here. 

 The specimens at hand present no evidence on this subject. The upper 

 or third series of plates can be diagrammed readily so as to conform to 

 the system worked out for Aethocystites scidptus, although the suture 

 lines are indicated only faintly. 



After studying specimen number 21815, just described from the 

 Walker Museum collections, other specimens of casts of the interior of 

 Lysocystites nodosus become readily intelligible. In the Walker Museum 



