CRINOID GENUS SCYPHOCRINUS 45 



genera, and as performing the same function; but it is certainly a long road 

 from a pinnule, usually regarded as a miniature arm, to such ponderous struc- 

 tures as the interradially located tubes of a form like Gilbertsocrinus typus 

 (N. A. Crin. Cam., pi. 14, fig. 1), built up of circular segments like a stem. 

 Nevertheless, their common origin is clear. 



In their earliest stages (Devonian and the base of the Lower Carbon- 

 iferous) the tubes were short (G. spinigerns, G. stellaris, and G. fiscellus (N. A. 

 Crin. Cam., pi. 15, figs. 3b, e, and 4; pi. 17, fig. 2a). In G. stellaris (deKoninck 

 and Lehon) we have a Rhodocrinns with the pores next to the arm-bases 

 extended into small tubes, of probably very few segments, without any ventral 

 groove ; they are at the interradial sides of the ray. In other European species 

 of Lower Carboniferous age — for example, G. calcaratns Phill. (N. A. Crin. 

 Cam., pi. 15, fig. 5) — the tubes have become large, but are still separate and 

 radially located. In the upper Burlington limestone (N. A. Crin. Cam., pi. 14, 

 fig. 1 ) the tubes attained a size which dwarfs all the other parts of the crinoid, 

 being more than twice as long as the extreme length of the calyx, and as thick 

 as the stem. Those from adjacent rays unite above the interradius, coalesce 

 by zigzag sutures for a distance of six or eight plates, beyond which they 

 separate and taper gradually to a point. Thus while of radial origin, the 

 tube where doubled is interradial in position. Each tube is perforated through- 

 out by a central canal, and not only is it composed of circular undivided seg- 

 ments like the ossicles of a column, but the joint-faces are often somewhat 

 striated like those of stems — there being no semblance of the articulation 

 belonging to arms or pinnules, nor of any ventral groove. The construction, 

 therefore, is that of a stem appendage rather than of an arm appendage. 



In the Keokuk limestone another curious change occurs (N. A. Crin. 

 Cam., pi. 16, figs. 1-6). The tubes from adjacent rays coalesce interradially as 

 before for a short distance and then separate; but instead of being com- 

 posed of undivided segments like a stem, they take on the construction of an 

 arm or pinnule, so that there is one plate (brachial) dorsally and two large 

 plates at the ventral side which interlock like covering plates. Of course, 

 where the tubes are joined the number of these elements appears doubled. The 

 tubes are longitudinally perforated by a fair-sized channel, but there is no 

 ventral groove. This change marks the end of the genus; and this type of 

 extreme aberrant growth of pore or pinnule is seen no more among the crinoids. 



It is evident that this extraordinary development from a pinnule having 

 a tendency to irregular growth must be interpreted as a pure case of hyper- 

 trophy in which the original character of the organ was wholly lost, followed 

 by an effort at recovery in the expiring struggles of the genus. And it is now 

 most interesting that a condition comparable to this, and which gives a clue 

 to the strange tendencies which might give direction to the impulse of hyper- 



