154 FOSSIL CRINOIDS. 



variations are thoroughly shown. The small size of the pinnules may be seen 

 in Figs. 4, 5; in the larger specimens they are either broken off, or cannot be 

 shown by cleaning in the hard matrix; only traces of them can be seen in Fig. 1. 



The ventral sac is remarkable for its great size and complicated structure. 

 It is built up of longitudinal rows of hexagonal plates similar to the anal plates, 

 with a median elevation connecting from plate to plate, and forming ribs extend- 

 ing the full length of the sac, and converging at the distal end like meridians of 

 longitude. The surface of these plates is completely covered with sharp and 

 deep transverse folds which completely obliterate the sutures, so that their 

 outline cannot be seen. The sac evidently extends beyond the ends of the arms, 

 and tapers to a narrow point, as shown by the imprint in the matrix in Fig. 1. 

 I have not been able to locate the anal opening in any of the specimens ; in this 

 group generally it was at the anterior side — the sac representing morphologi- 

 cally a highly developed interbrachial structure extruded through the anus — 

 and from what appears in the next species I think it was near the base of the sac. 

 The column, as usual in the genus, is round, composed of very short, uniform 

 columnals, which increase very slightly in length distally; and there is a very 

 gradual taper at the proximal end. 



This species is confined to the Upper Burlington Limestone, and is sharply 

 distinguished from P. macropleurus of the Lower by its very long brachials. P. 

 notabilis, also from the Lower Burlington, is a similar elongate form, but wholly 

 lacks the striae along the ridges, the best preserved specimens failing to show 

 any trace of them. In surface characters of the calyx it has an extraordinary 

 resemblance to P. radiatus of Austin, but the arms and ventral sac of that 

 species, as figured in Mon. Rec. and Foss. Crin., PI. X, Figs, la, h, look more 

 like those of Parisocrinus. 



Types. The type specimen, formerly in the collection of Dr. C. A. White, 

 cannot be located. The species was exceedingly rare in the early days of collect- 

 ing at Burlington, but Wachsmuch was quite familiar with the type, and labeled 

 specimens of his own after direct comparison with it. Nearly all of my speci- 

 mens above mentioned were found long afterwards in two colonies, and from some 

 of these the figures given here are made. 



Horizon and Locality. Upper Burlington Limestone. Burlington, Iowa. 



