134 SYNOPTIC COLLECTION. 



the base the vesicles are larger and there is a tendency 

 towards forming tabulae. During periods of rest or com- 

 parative inactivity the vesicular mass becomes more or 

 less dense and apparently a cup is formed. For this 

 reason there is a succession of cups, representing not differ- 

 ent zoons that have budded but different degrees of activ- 

 ity in forming the skeleton. If each were a bud, one 

 ought to find the epitheca extending down into the cup, 

 which is not the case. Still there are some specimens 

 which appear to be made of the skeletons of two zoons, 

 and as the epitheca is continued on the outside it is diffi- 

 cult to give an explanation. In most specimens the epi- 

 theca is worn off, but when preserved it shows distinct 

 concentric ridges. 



The operculated corals seem to be related to the Cysti- 

 phyllidae although they are more specialized in structure. 



One of the forms whose affinities have so far baffled 

 naturalists is the Cretaceous Barrettia monilifera Wood- 

 ward (PI. 213, fig. i, greatly reduced). Its shape and 

 general appearance would place it with Calceola and the 

 other operculated corals, but in structure it is different 

 from any fossil so far discovered. According to Wood- 

 ward, who first described the specimen (1862), it belongs 

 to the Rudistae, a group of molluscs to which also Hip- 

 purites and Radiolites (PL 428, figs, i, 2) belong. This 

 view was based upon the fact that in Woodward's speci- 

 mens Barrettia possessed a bivalve shell. According to 

 Whitfield (1897) Barrettia is more nearly related to corals 

 than to molluscs. 



The visceral cavity occupied the center (PI. 213, fig. 2, 

 longitudinal section ; fig. 3, cross section). Below, it was 

 divided by transverse partitions (fig. 4). Radiating from 

 the central cavity were lines of vertical tubes or monili- 

 form rays (figs. 3, 4), and close by it was a larger tube 

 divided by transverse walls (fig. 4 a). 



The spaces between the radiating rows of tubes were 

 filled with four-walled tubes which were also divided 



