150 SYNOPTIC COLLECTION. 



The central mouth was concealed by oral plates in the 

 living animal, and the ambulacra were also covered. Co- 

 daster like all Blastoids is without arms, but short pinnules 

 are attached to the ambulacra. 



In another genus, Orophocrinus (PI. 261), the numerous 

 hydrospiral slits are reduced to ten slits, two on each side 

 of an ambulacrum (PI. 261, s). In this figure the cover- 

 ing plates are seen over the ambulacra. 



If we suppose the hydrospires crowded under the ambu- 

 lacral areas and these slits shortened till only an opening 

 is left at the top, we have the condition found in the fol- 

 lowing more specialized Blastoids. 



Pentremites godoni Shum. (No. 262), is a stalked Blas- 

 toid, but the stem is so short and small that it is rarely 

 preserved. The body in this genus, as in all Blastoids of 

 its group (Pentremitidae), becomes constricted and the 

 inner portion of the basal plates helps to form the upper- 

 most disc of the stem. The bas\ls and radials are well 

 seen in No. 262, especially in the middle specimen (b) 

 in the lower row, and the upper right hand specimen 

 (e). Each of the five ambulacra consists of two parts, the 

 lancet-shaped portion in the middle which is made of 

 many small plates, and the side pieces or plates. Near 

 the outer edge of the lancet plate there is a row of sock- 

 ets where the long delicate pinnules were attached. The 

 food was caught by these pinnules and carried by cilia in 

 the transverse channels to the median channel and thence 

 to the mouth which was in the center of the oral disc. The 

 ambulacral groove is said to have been covered by plates, 

 but these are not seen in any of the specimens in the 

 Society's collection. 



On the outer edges of each ambulacrum is a row of 

 holes (No. 262, a, b, d, e) for admitting water to the tubes 

 or hydrospires inside. The latter open at the top in the 

 five holes or spiracles around the mouth (No. 262, c, d). 

 In reality four of the spiracles are divided by a partition, 

 while the largest one is divided twice. Of the eleven 



