VIII. Cyathocnnus. 225 



that the ventral sac of the Cyathocrinid* generally is not 

 perforate. It is ])o.ssihle that they were formerly misled by 

 the erroneous reference 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 j)lates. 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. acinotubus (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 ambulacrals and interambu- 

 lacrals, though portions of the deltoids are still seen peeping 

 out from beneath the interambulaerals. 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, wiiich 

 are sometimes large and fairly regular, sometimes small and 

 irregular, orals or merely large ambulacrals? 



EXPLANATION OF PLATE XIU. 



Cyathocrmus acinotttbus. 



Fig. 1. 57480, B. M. A crown witli portion of stem, seen from the right 



side. Tlie ventral sac seen crossing between the arms. Drawn 



with the camera by Mr. IluUick. (Nat. size.) 

 Fig. 2. E 1450 B. M. Crown seen from posterior. Drawn with the 



camera by Mr. HoUick. [Xat. size.) 

 Fig. 3. a62(5, 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/52(3, Woodwai'dian Museum. Cup seen slantwise from below, 



showing projection of radials. One very pentagonal cohimnal 



seen inserted in the IBB circlet. Fi-om a drawing by Mr. E. 



Wilson. (Nat. size.) 

 Fig. 5. a/526, Woodwardian Museum. Cup seen from posterior, showing 



X and It. From a drawing by Mr. E. \\'ilson. (Nat. size.) 



