714 



Keport of the State Geologist. 



the basal cup could be discerned ©nly in exceptional cases (cf. fig. 12), it at 

 once becomes apparent in somewhat larger specimens that the appendage 

 differed only in size from the organ previously described. Figures 11 to 34 

 illustrate sufficiently the various aspects, and also the somewhat differing size 

 which the appendages show according to their state of preservation and the 

 direction and amount of their compression. The specimens w hich exhibit the 

 underside of the base (cf. PI. IV, figs. 12, 13, 21, 22) show plainly its radiated 

 surface. Others give lateral views oi the appendage (cf. figs. 4, 9, 10, 11, 18, 

 19, 23, 24, 26, 27, 28). Of these the original of figure 10 is especially inter- 

 esting. This has apparently preserved the protruding interior cup; that of 

 figure 18 plainly shows the ring-like thickening of the basal part of the exte- 

 rior cup. Some specimens even present sections of the base (cf. figs. 8, 16, 

 17, 32, 33), While the majority of the appendages, which appear as solid, 

 smooth discs (cf. figs. 5, 7, 29, 30) evidently expose the upper side to view. 



While most specimens terminate in triangular outlines, undoubtedly 

 formed by the apertural processes in an expanded state, numerous shells 

 present at first sight a somewhat puzzling aspect by the appearance of a wel] 

 defined, often darker, and somewhat projecting square plate or of a corre- 

 sponding impression, at the distal end (cf. PI. IV, figs. 13, 19 to 22, 32). As in 

 sev eral specimens the composition of this plate of four triangular pieces could 

 be observed (cf. figs. 1!), 21, 22), it may be safely inferred that, by an 

 obliquely lateral compression, the summit aspect of the fossil has been pre- 

 served and all four apertural processes are visible. A complication of this 

 aspect is often caused by the appearance of a central circular impression 

 which is surrounded by one or more concentric ridges (cf. fig. 20). The 

 writer is disposed to regard the latter as parts of the internal shell-wall which 

 have been pressed through the apertural plates. This assumption pre- 

 supposes that the interior section of the shell was not quadratic but circular, 

 a supposition which seems to be supported by the occurrence of examples 

 which show a well defined circular spot in the center free from carbonaceous 

 matter (cf. fig. 23). 



Indications of a surface sculpture are extremely rare in the great 

 majority of infantile specimens. All that can be seen in the smallest speci- 

 mens is. now and then, a series of distant transverse furrows (cf. PI. IV, figs, 6, 

 14, 24. 2.")). w hich would seem to belong to a deeper, transversely fractured 

 layer, such as has been observed in the original of Plate III, figure 4. The 

 original of figure 26, Plate IV, shows a fine longitudinal striation and that of 

 figure 27 exhibits at the distal end distinct longitudinal ridges and slight indi- 



