546 



GEOLOGICAL SURVEY OF IOWA. 



Fia.66. 



The form and proportions of parts in this species are 

 shown in the accompanying diagram. 



These illustrations suffice to show the characteristics of the genus, which begins its 

 existence, so far as known, in the Burlington limestone, and continues to the Chester 

 limestone, becoming far more abundant and extravagant in its forms in the latter 

 period. 



FIG. 67. 



The diagram fig. 67 illustrates the interior of the calyx 

 of Zeacrinus depressus (TROOST), of which fig. 66 is the 

 exterior. The basal plates (1), which are extremely small 

 in the exterior basal cavity, are, on the inner side, much 

 larger than the subradials, and are truncated at their 

 upper extremities by the first radial plates. The subra- 

 dial plates (2) present, on the interior, an extremely 

 small quadrangular face compared with the elongate spa- 

 tulate form of the exterior, and, instead of joining at their lateral edges, are entirely 

 separated from each other by the basal and first radial plates. The first radial plates 

 (3) are short, hexagonal in form, with the lower side very narrow, as well as the 

 two adjacent sides. The inner face of the plate is the space included in the lines sur- 

 rounding the figure (3) : the remaining portion is the thickened articulating margin, 

 the central space of which, marked a, is higher than the inner or the outer areas 

 marked 6 and c respectively, the latter having been occupied by a softer yielding sub- 

 stance; while the plates have been in actual contact with the succeeding range along 

 the small central area connected by a tendinous fibre, as is indicated by the little 

 oval pit or cicatrix on the inner edge of this part of the plate. 



The Poteriocrinus maccoyanus of DE KONINCK and LE HON possesses the charac- 

 ters of ZEACKINUS, in the concave base and minute basal plates hidden by the column, 

 as well as in the character of the anal plates. 



The Cyathocrinus maniformis = Poteriocrinus maniformis (YANDELL & SHUMARD), 

 which is probably identical with the Cyathocrinus gradlis ( TROOST), belongs to 

 ZEACRINUS, as well as the Cyathocrinus Jlorealis = Poteriocrinus Jlorealis of the same 

 authors. 



The species form a very characteristic group, and are readily distinguished by 

 general form and habit from the ordinary forms of Poteriocrinus of the same strata. 



