494 FRANK SPRINGER 



On the ventral side we might expect modification in the same 

 way that we see in Antedon, viz., a growth of perisome between 

 the radials and orals, gradually separating the latter plates from the 

 raidals and carrying them inward toward the center. This would 

 give the condition found in Onychocrinus and Taxocrinus; but the 

 cases in which these structures are preserved in the fossils are too few 

 to enable us to trace any of the successive changes. 



a) The first of the above modifications of the dorsal side has been 

 already discussed, both in this and my former paper; and it has 

 been shown that there are to some extent parallel modifications of 

 this character in the two families as formerly defined, which might 

 form the basis of some lesser groups. But the radianal was early 

 eliminated in this group, being known only beyond the Silurian in 

 two Devonian species, and modifications on this line do not cut much 

 figure in the delimitation of the families. There are still some 

 troublesome questions of interpretation among the Swedish species, 

 owing to irregularities. Several of them are represented by only a 

 single specimen, and this in some cases abnormal. These cases will 

 be left for consideration hereafter in greater detail, in the hope also 

 that the discovery of further specimens will throw new light upon 

 them. 



b) The generic distinctions based upon the other modifications of 

 the posterior, or anal, area, coming under the second category, are 

 really very striking — more so than can be well expressed in terms of 

 brief analysis. Aside from the matter of the radianal, there are two 

 plans of structure of the anal area, which run side by side from the 

 Silurian to the Carboniferous. They start with the primitive anal 

 plate of our supposed ancestral form, and diverge upon the two lines 

 I and 2 indicated above. 



I. The first of these plans is one which represents a solid support 

 or backing of an anal tube. It is marked by a vertical row of strong, 

 rounded plates, originating on the posterior basal, rising with a very 

 gradual taper to a considerable height between the rays, and having 

 the appearance of a small, rounded arm. It is connected with, or 

 rather seems grown into, the pliant integument of small, irregular 

 plates which formed the perisome, or ventral covering in this group. 



