136 FOSSIL CRINOIDS. 



to a rim or elevation about the central canal. In the upper center of the dorsal 

 ligament fossa there is a more or less sudden depression, the ligament pit, just 

 under the transverse ridge, which may be more or less excavated above it. 

 Along the transverse ridge there may be seen a narrow line of transparent, 

 condensed, calcareous matter representing the apex of the fulcra! ridge. This 

 is the condition in the young, and generally in adult individuals; but in many 

 species there comes with age an increasing stiffness in the joints; the sculpture 

 of the joint face gradually becomes obsolete or entirely obliterated, and a plane, 

 or more or less curved, union of almost undifferentiated articular faces results. 

 Some of the "Actinometras" have progressed far along these lines. In the 

 obsolescence of the articular faces a curious modification is often seen; with the 

 disappearance of the transverse ridge radial crenellae appear along the dorsal 

 margin of the dorsal ligament fossa, and the transverse ridge, or what remains 

 of it, becomes corrugated, or tuberculated. 



In the Crinoids, whenever a union, through old age or otherwise, becomes 

 so close as to preclude motion, these crenellae always begin to form; peripheral 

 at first, they gradually spread inward until they sometimes even reach the 

 central canal, so that, to all appearances they are syzygial ; but in young speci- 

 mens their entirely different aspect can be readily made out. In the genus 

 Comatula (Solaris group of P. H. Carpenter) all gradations are seen in the "inter- 

 costal" articulation; it may be a typical synarthry; the ligament fossae may 

 become so shallow as to result in a practically flat joint face, without the longi- 

 tudinal ridge; or crenellae may creep inward so that the joint face appears 

 practically a perfect syzygy. The muscular articulations, as well as the syn- 

 arthries may undergo a similar transformation. 



Thus we see that in the living Crinoids there are a variety of stages, or con- 

 ditions, as to the structure of these parts, some or all of which may be expected 

 to have become established and run their courses for whole groups, in palaeon- 

 tological epochs. The fully organized joint face, as above described, represents 

 a generalized type which would tend to persist, like the simple calyx of the 

 Inadunata, from earliest times to the present. The undifferentiated joint face 

 is a highly specialized condition, such as, if established in a given group, would 

 tend to indicate a definite limitation of its geological range, and its early extinc- 

 tion. Recurring to the statement of Wachsmuth and Springer above cited, it 

 will be seen that the two conditions, adolescent and senile, arc represented 

 palaeontologically by our two divisions, Poteriocrinidae and Cyathocrinidae. 



Much stress has been laid upon this matter of articulation by many authors, 



