Keyes. ] 236 [Oct. 19, 
opment. The nodes of certain plates are much more pronounced 
than others, and are more or less confluent, forming a conspicu- 
ous stellate figure, the points of which are directed interradially.. More 
than one-half of the Crawfordsville specimens of this species have resting 
on the vault a Platyceras—usually P. eqguilaterum Hall. The specimens 
illustrate well the adaptation of the apertural margin of the shell to the 
irregularities of the crinoidal surface, for it is clearly observable, as was 
first pointed out by Meek and Worthen, that the contact of the gasteropod 
shell and crinoid is not the result of accidental pressure, but that the mol- 
lusk adhered to the surface of the crinoid for a considerable period, as is 
shown by the sinuosities of the peristome corresponding exactly to the 
inequalities of the surface beneath. In young shells the sinuosities of the 
apertural margin are comparatively much more pronounced than in older 
individuals. Many of the latter exhibit much irregularity in the lines of 
growth, which might at first appear to be due to a change of situation, 
but closer inspection shows that this is not the case.. When the plates of 
the vault are nodose, as in Ollacrinus tuberosus, the lines of growth in old 
shells, contrary to the more usual manner among gasteropoda generally, 
are far from being even approximately parallel to one another ; and in the 
margin of the shell a sinus caused by a nodose plate at one period of 
growth may be represented in the next by a projecting lobe which had 
extended into a deep depression between the nodes of two contiguous 
plates. From an examination of the impressions made by the calyptrean 
shell on the vaults of Strotocrinus and some other genera, it appears that 
the anterior end of the Platyceras is always stationary at the margin of 
the anal opening, and that as the growth of the shell continues the poste- 
rior portion is removed farther and farther from the ventral aperture in 
the crinoid. Platyceras equilaterwm, when stationed on the flat vault of 
Ollacrinus, has the spire much more depressed, and the aperture compar- 
atively much larger than is usual with this species. 'Tbis fact is also in 
accordance with observations among the living representatives of a closely 
allied genus, Capulus, in which it has been found that individuals attached 
to flat surfaces have the shells much more depressed than those that 
have settled on convex bodies. If Platyceras wqutlaterum was entirely 
sedentary in its habits, as there is reason to believe, it would seem, from 
the diverse forms, that its habitat comprised a variety of objects other 
than crinoids. Indeed the shell of this gasteropod is extremely variable 
in form, some specimens being nearly hemispherical with a small spire 
subcentrally located, while others are nearly flat and the spire projects far 
beyond the posterior margin of the aperture. Of the many examples of 
crinoids with attached Platycerata only two instances have been noted in 
which these gasteropods are associated with Actinocrinus, or other genera 
having a prolonged anal tube—the so-called ‘‘ proboscis.’’ Meek and 
Worthen* knew not asingle case of this kind, and state that it is ‘‘prob- 
ably due to the fact that in species with a ‘proboscis’ there is much less 
* Geol. Ill., Vol. v, p. 335. 
