308 



PHYLUM ECHINODERMATA. 



same time restricted in area, and often surrounded by a raised rim. 

 While normal pore-rhombs are usually found on all or most of the plates, 

 pectini-rhombs only occur on a few plates (Fig. 219) in definite posi- 

 tions. The grooves of a pectini-rhomb are frequently filled up or bridged 

 over near the suture, so that the rhomb as seen from outside appears 

 to consist of isolated halves. These halves, however, remain connected 

 within the mesostereom. 



FIG. 218. Pore-rhombs (a) of Echinosphaera, (b) of Caryocrinus (magnified, from Zittel). 

 The left half of a is abraded, so that the canals appear as open grooves. 



It appears that the canals, whether of the ordinary pores or of the 

 pore-rhombs are typically in the mesostereom, and do not open on the 

 surface, being covered by the epistereom and hypostereom respectively, 

 though if these layers are absent through weathering or other cause, the 

 canals appear in some cases to be open. It is possible that they are due 

 to tracts of stroma containing blood spaces traversing the stereom, and 

 Mr. Bather has suggested that the canals of the pore-rhombs are develop- 



no. ZlQ.Callocystites Jewetti Hall. Upper Silurian, Lockport, New York. A from the 

 side (natural size). B ambulacral grooves and two pectinated rhombs rh. an anus ; 

 g genital opening ; o mouth. 



ments from foldings of the mesostereom, such as exist in many Crinoids, 

 being due to the natural tension of the stroma fibres in the integument as 

 it becomes calcified. These foldings are covered towards the surface by 

 the secondary deposition of epistereom and hypostereom. When the 

 epistereom was very thin or absent, as appears sometimes to have been 

 the case, or when it has been removed by weathering, these grooves of the 

 mesostereom appear to be open and their edges project on the surface of the 

 plates as ridges. Whether this view of the real nature of the canals is 



