Keyes. ] 238 [Oct. 19, 
celatus Phillips, from the Upper Silurian of Dudley, England, the vault 
is composed of a large number of small plates, and the anus is subcen- 
tral. In Platycrinus and Hucladocrinus the vault is more or less hemi- 
spherical with the simple anal aperture lateral in position. Platycrinus 
hemisphericus M. and W. isa common species in the Keokuk shales of 
‘rawfordsville, Indiana, and like Ollacrinus from the same locality a large 
proportion of the specimens have attached Platycerata—usually P. infun- 
dibulum M. and W., though not unfrequently P. equilaterum Hall is also 
found associated with this Platycrinus. The former is a straight, elongate- 
conical shell with numerous longitudinal plications ; while the latter is a 
mote or less depressed, subspiral form, with a very large aperture. The 
attachment of these gasteropods to Platyvrinus is somewhat different from 
that of the other genera mentioned, being at the side between the arms, 
instead of on the vault. As the growing shell increased in size it encom- 
passed the bases of two postero lateral arms, and in some cases the poste- 
rior portions of the univalve even extended to and enclosed the stem of 
the crinoid. Hucladocrinus, from the Burlington limestone, presents the 
same general features as Flatycrinus in the attachment of Platycerata ; 
and it is worthy of note that this gasteropod from the Burlington is 
remarkably similar to P. infundibulum from the Keokuk. In Arthroacan- 
tha, Platyceras erectum Hall is found on the vault. Hinde* mentions two 
species of Platyceras associated with this crinoid, P. erectwm Hall and P. 
dumosum Conrad. In Pterotocrinus the vault is entirely different from 
those of the other genera here mentioned. It is much elevated, being two 
or three times as high as the dorsal cup. The first radial dome plates are 
produced into monstrous alate processes, leaving a small summit which is 
perforated for the anus. The anal opening is usually covered by a very 
small Platyceras. In a number of examples the summit of this crinoid 
became too small for this little gasteropod, and the margin of the growing 
shell has followed closely the depressions between the radial dome plates, 
which have given rise to large deep sinuosities in the apertural margin. 
Oromyocrinus simplex Trauts., from the subcarboniferous of Russia, sup- 
ports a broad subspiral Platyceras described as Oapulus parasiticus Trauts. 
Trautschold} in describing the crinoid calls attention to a cylindrical pro- 
cess lying above the anal plate, and to a gasteropod frequently found 
attached to this region of the calyx, he then adds: ‘ Es ist nicht unmog- 
lich, dass der oben beschriebene cylindrische Processus der Analplatten 
zum Schutz gegen diese Verfolger des Cr. simplew aufgebaut ist.’”? The 
‘cylindrical process’? here referred to is manifestly a ventral sac, and 
both its structural and functional character were entirely misunderstood. 
The occurrence of Platycerata attached to some other species of crinoids 
has been noticed, but opportunity for personal examination has not been 
offered. Yandell{ mentions that gasteropods are sometimes enclosed in 
* Ann. and Mag, Nat. Hist., March, 1885, p. 172. 
+Die Kalkbriiche von Mjatschkowa, p. 119. 
} Proc. A, A. A. S., 1851, p. 234. 
