1888, ] 237 
[Keyes. 
room for attachment to the vault.”’. If the gasteropods of this genus sub- 
sisted upon the excrementitious matter from the crinoid the reason is 
obvious for their non-occurrence on species having a prolonged anal tube. 
In one of the cases referred to the Platyceras was just visible between two 
of the arms of the crinoid, but the matrix was too compact to permit of 
removal, and in the other, A. verrucosus Hall, the molluscan shell was at 
the base of the anal tube, which, however, appeared to have been injured. 
It is very probable that on account of this deformity there was an addi- 
tional opening at its base. 
But few illustrative instances are furnished by Physetocrinus. In P. 
ventricosus Hall the dome is low, hemispherical, but the univalve asso- 
ciated is remarkable for its comparatively large size, covering nearly the 
entire vault. Strotocrinus presents numerous interesting examples. S. 
regalis Hall, from the Burlington limestone, has the vault relatively very 
large and flat as in Ollacrinus, but the dome plates are small and nearly 
plane. Like in the Ollacrinus from Crawfordsville, Ind., the undeter- 
mined Platyceras found on this crinoid is very much depressed, the aper- 
ture ample and nearly circular in outline. In several specimens the uni- 
valve has been carefully removed, revealing a series of concentric impres- 
sions which, as has been previously stated, mark the different stages in 
the growth of the shell. Perhaps one of the most notable examples is an 
undescribed Dorycrinus* from the Kinderhook of Marshall county, Iowa. 
In this species the dome is convex with broad radial elevations which 
become more pronounced toward the arm bases. ‘On the posterior side of 
the calyx there is a prominent ridge, at the top of which is situated the 
anal opening, just below the central dome plate—the latter being rather 
large and produced into a high conspicaous node. The arms are twenty 
in number, equally divided among the rays, and equidistant from each 
other, forming a close, almost continuous circle around the periphery of 
the vault. The associated Platyceras is of the P. infundibulum type, but 
instead of being attached laterally between the two posterior arms—as in, 
the case of the Crawfordsville species of Platycrinus—it is stationed 
directly on the vault, with the anterior portion of the shell over the anal 
opening. In this position the mollusk has its anterior extremity directed 
toward the posterior side of the crinoid, instead of the usual opposite 
direction ; it also covers the subspinous process of the central plate, and 
the apertural margin reaches to the arm bases on all sides except the two 
postero-lateral. When thus covering nearly the entire vault the de- 
pressed interradial areas give to the aperture of the shell a marked quin- 
quelobate appearance. 
The Platycrinide present both numerically and structurally the finest 
series of illustrative examples, but they are unequally distributed among 
three genera: Marsupiocrinus, Hucladocrinus and Platycrinus—the typical 
genus furnishing the large majority of the specimens. In Marsupiocrinus 
*This Dorycrinus has been defined by Wachsmuth and Springer, in the eighth volume 
of the Nlinois Geological Survey, now in press. 
