BASAL PLATES IN CRINOIDEA CAMERATA 547 



and along the right-posterior radius in the basal cycle, and it is to 

 the latter zone that especial attention is called. Wachsmuth and 

 Springer have noted this zone of potentiality in but one instance 

 (description of Fig. I, No. 7, of this paper), although it seems the 

 only zone of potentiality in the basal cycle which can logically be 

 accounted for. Pressure from the end of the developing hind-gut 

 must from the necessity of its position be directed obliquely to 

 the right against the posterior interradius, and any lateral pres- 

 sure of the gut must be directed against the right side of the 

 calyx, the two combining to shove the right side outward and 

 away from the left side. Thus the stress produced by this shove 

 would naturally fall along the right-posterior basal suture, as this 

 is the nearest suture or plane of expansion adjacent to the posterior 

 interradius within the zone of pressure exerted by the hind-gut. 



ORIGIN OF THE ANAL PLATE 



Comparison of the cup and tegminal structures in the Bato- 

 crinidae and Platycrinidae shows that a very long period of time 

 must have elapsed, or very rapid evolution have taken place, before 

 such a highly specialized form as Tanaocrinus could have originated 

 from any of the early Platycrinidae. Since Tanaocrinus is an early 

 Silurian (Richmond) genus, relationship can be established with 

 the Platycrinidae only through Ordovician or pre-Ordovician ances- 

 tors; therefore, if Tanaocrinus is related to the Platycrinidae, it 

 must have been derived from a form having a simple, pentagonal, 

 five-basal cup. The primary step in the evolution of this form 

 into Tanaocrinus would be the introduction of the anal plate 

 into the radial cycle, thus giving to the cup its hexagonal outline, 

 and inducing in the basal-plate cycle a remarkable series of modifi- 

 cations. The questions then arise : How do we know that the anal 

 was a secondary and not a primary plate ? At what period in the 

 ontogenetic development of the Camerata was it interpolated in 

 the radial cycle? Where did it originate? And what changes 

 followed its interpolation ? - It is generally agreed that the anal 

 plate in the hexagonal Camerata is of secondary origin. If this is 

 true, the statement just made concerning the ancestry of Tanao- 

 crinus is undeniable, for by eliminating the secondary plates of 



