Keyes. ] 232 (Oct. 19, 
plants; that in the palexocrinoids* the mouth is subtegumentary, instead 
of being externally visible as in the recent crinoids, and that the aperture 
in the vault is the anal opening. With these considerations in view the 
association of these calyptreean shells and paleozoic crinoids assumes an 
entirely different phase from that originally entertained. 
The genus Platyceras was founded by Conrad} in 1840 for a paleozoic 
group of gasteropodous shells ‘‘suboval or subglobose, with a small spire, 
the whorls of which are sometimes free and sometimes contiguous ; the 
mouth generally campanulated or expanded.”’ Hitherto these fossil shells 
had been referred to the genus of modern mollusca Capulus, proposed by 
Montfort} in 1810; or to Péileopsis, founded by Lamarck§ in 1822 for the 
same group. Conrad’s name for this fossil group was not, until within 
the past few years, generally accepted, preference having been given by 
most European writers, and also by some American authors, to Acroculia 
of Phillips, || notwithstanding the fact that the type of Phillips’ genus was 
a typical form of Platyceras. Some European writers even now question 
the propriety of generically separating Platyceras from Capulus, and con- 
tinue to describe specimens belonging to the former group under the latter 
genus. Among living Calypiraide it has been observed that both color 
and form are dependent upon individual environment and hence in forms 
of the same species there may be many varietal phases. It has further 
been noted that the majority of the members of this family attach them- 
selves while yet quite young to stones and shells of other mollusca, and 
having found a suitable situation seldom, if ever, remove from the spot 
where first they became stationed. The character and contour of the sur- 
face on which they have settled would therefore determine to a great 
extent the form and outline of the apertural margin. The evidence here 
presented manifestly proves that the fossil representatives had, with per- 
haps one exception, similar habits and were subjected to like conditions 
of environment. 
I. History. 
1843. Among the first to note the intimate connection of molluscan 
shells and crinoids were the Austins, {[ who, in their description of Pote- 
riocrinites crassus, discuss at length the ‘‘carnivorous’’ propensities 
of the crinoids in general. Particular mention is made of Producti being 
the victims of the ‘‘predatory explorations’’ of the Potertocrint; and 
there is reference also to ‘‘other crinoids having been found with uni- 
valves inclosed within their rays, in such a position as to leave but little 
*Tt must be bornein mind that paleocrinoidea and palwxozoic crinoideaare not coexten- 
sive terms. 
+Ann. Rept. Paleo. N. Y., p. 205, 1840. 
{Conch Syst., Vol. ii. 
2His. Nat. des Animaux sans Vertébres, 1815-1822. 
|| Palee. Foss. Cornwall, p. 93, 1841. 
{ Monog. Recent and Fossil Crinoidea, p. 73. 
