1888. ] 235 [Keyes. 
1881. Wachsmuth and Springer* mention a gasteropod generally cov- 
ering the anal aperture of Pterotocrinus depressus. 
1883. Julien Fraipont,f in his Recherches sur les Crinoides du Fam- 
ennien de Belgique, mentionsa Capulus affixed to aspecimen of Melocrinus 
globosus Dew., and in such a position as to indicate that the Capulus did 
not form the food of the crinoid. 
1885. Hinde, describing a new species of crinoids with articulating 
spines, remarks that three out of eleven specimens discovered of Hystri- 
crinus carpentert (= Arthroacantha punctobranchiata Williams) have a 
Platyceras resting on the vault, which it nearly entirely covers ; and there 
is no doubt that the gasteropod derived its support from the materials 
rejected by the crinoid. 
1888. Keyes,§ in the American Journal of Science, gives a brief sum- 
mary of the sedentary habits of Platyceras, and mentions six species of 
crinoids having gasteropods attached. 
II. DescripTion OF SPECIMENS. 
It has been intimated elsewhere that the direct paleontological evidence 
of the sedentary habits of the mollusca belonging to the genus Platyceras 
is found chiefly in the attachment of gasteropod shells to the calyces of 
certain species of paleozoic crinoids. The diverse structural differentia- 
tion of the crinoidal vault and the well-defined surface ornamentation in 
the various genera, presents, in those species in which the association of 
the univalves has been observed, determinate features for an adequate 
consideration of the variability in the apertural margin of the calyptrean 
shell. Numerous illustrative examples of the following species of crinoids 
have been examined :| Ollacrinus tuberosus Lyon and Casseday, O. typus 
Hall, Actinocrinus verrucosus Hall, Physetocrinus ventricosus Hall, Stroto- 
erinus regalis Hall, Dorycrinus immaturus{, Wachsmuth and Springer, 
Marsupiocrinus celatus Phillips, Hucladocrinus millebrachiatus Wachs- 
muth and Springer, Platycrinus hemisphericus Meek and Worthen, <Ar- 
throacantha punctobrachiata Williams, Pterotocrinus acutus Wetherby, 
and Oromyocrinus simplex Trautschold. 
In Ollacrinus the vault is comparatively large, nearly flat, with the anal 
opening eccentric, and the plates are more or less convex or nodose. In 
some specimens of 0. tuberosus L. & C. from the Keokuk of Crawfords- 
ville, Indiana, this nodosity of the ventral plates reaches a high devel- 
* Revision Paleeocrinoidea, pt. ii, p. 90, 1881. 
+ Annals Soc. géol. de Belg., t. x, Mémoires, 1883, p. 62. 
t Annals and Mag. Nat. His., Aono 1885, p. 172. 
2Am. Jour. Sci. (8), Vol. xxxvi, p. 269, 1888. 
| The observations herein noted are based principally upon numerous exemplifications 
in the extensive and unrivaled series of paleozoie crinoids in the collection of Messrs. 
Wachsmuth and Springer, to whom sincere thanks are tendered for free access to their 
cabinets. 
{It may be stated that this species has been figured and describedin the eighth vol- 
ume of the Ilinois Geological Survey, now in press. 
