VIII. Cyathocrinus. 225 
that the ventral sac of the Cyathocrinide generally is not 
perforate. It is possible that they were formerly misled by 
the erroneous reterence to Cyathocrinus of many species of 
Gissocrinus, Botryocrinus, and such forms in which the 
ventral sac often appears at first glance to be provided with 
slits between the edges of the plates. At any rate none of 
the sacs of the Silurian Cyathocrini that have come under my 
observation appear to possess either pores or slits. Decep- 
tive appearances are sometimes produced by weathering, as 
described under C. actinotubus (p. 220); and sometimes the 
edges of folded plates are filled with matrix which everyone 
does not take the trouble to clear away. 
The Tegmen.—As none of the British Silurian specimens 
show the tegmen it is advisable to defer discussion of the 
many important problems presented by it. Original drawings 
of two Carboniferous specimens are, however, given (p. 211) 
in illustration of the description of the genus. ‘The one 
(fig. 6) shows the deltoids and the madreporite surrounding 
the peristome ; the other (fig. 7) shows how both deltoids and 
peristome may be covered by ambuiacrals and interambu- 
lacrals, though portions of the deltoids are still seen peeping 
out from beneath the interambulacrals. The questions to be 
decided are these:—What are the true homologies of the 
plates here called deltoids? Is the madreporite serially 
homologous with the deltoids? Or is the posterior deltoid 
represented by two plates, one on either side of the madre- 
porite? Are the plates that cover over the peristome, which 
are sometimes large and fairly regular, sometimes small and 
irregular, orals or merely large ambulacrals ? 
EXPLANATION OF PLATE XIII. 
Cyathocrinus acinotubus. 
y 
Fig. 1. 57480, B. M. A crown with portion of stem, seen from the right 
side. The ventral sac seen crossing between the arms. Drawn 
with the camera by Mr. Hollick. (Nat. size.) 
Fig. 2. % 1450 B.M. Crown seen from posterior. Drawn with the 
camera by Mr. Hollick. (Nat. size.) 
Fig. 3. a/526, Woodwardian Museum. <A small cup, seen from below, 
with one or two columnals attached. Note large size of IBB. 
From a drawing by Mr. Edwin Wilson, artist to the Cambridge 
Engraving Co. (Nat. size.) 
Fig. 4, a/526, Woodwardian Museum. Cup seen slantwise frum below, 
showing projection of radials, One very pentagonal columnal 
seen inserted in the IBB circlet. From a drawing by Mr. E. 
Wilson. (Nat. size.) 
Fig. 5, 4/526, Woodwardian Museum. Cup seen fronr posterior, showing 
wand /é. From a drawing by Mr. E. Wilson. (Nat. size.) 
