MONOGRAPH OF THE EXISTING CKINOIDS. 215 



. As a result of the invariable occurrence of a proximalc, or one or more equivalent 

 structures, in the columns of the mesozoic and later crinoids (excepting in the 

 Encrhiidic, which in this as in other respects agree with pahrozoic forms, and in the 

 Plicatocrinidie), the varied shape of the column, which may be circular, elliptical, pen- 

 tagonal or stellate in section, the variation in the attachment, which may be by rariicu- 

 lar cirri, by a terminal stem plate, by a solid welding, or absent altogether, and the 

 enormous variability in the columnar growth, this being in some types, as in the pen- 

 tacrinites, excessive, and in others, as in Thiolliericrinus, abruptly reduced, while occa- 

 sionally, as in Marsupites and Uintacrinus, it is absent altogether, or, as in the other 

 comatulids, ceases abruptly before maturity is reached, the column in these tA-pes 

 comes to present the most rehable characters for broad systematic differentiation. 

 In the palaeozoic forms, where the columns arc, with rare exceptions, of a uniform t3-pe 

 and composed of a series of similar columnals, the variations in calyx structure are 

 of deep significance, far outweighing the characters offered by the column in system- 

 atic value; but in the later forms we see at once that in general the variations in 

 calyx structure are the direct result of the mechanical factors called into play by the 

 variations in the column. Thus as in the mesozoic and later types the calyx struc- 

 tiu-e is entirely dependent upon the structm-e of the column and has no special sig- 

 nificance other than illustrating methods of meeting various types of stresses in- 

 duced by the several types of stems, we are led to delimit our liigher. groups in terms 

 of column structure, passing over the vagaries of the calices, which are quite depend- 

 ent upon it. 



In the t^'pical crinoid column there may be recognized three distinct regions 

 each of which inclutlcs a different tj'pe of columnal from the other two; these 

 three regions are (1) the distal, (2) the middle, and (3) the proximal. The distal 

 region includes the terminal stem plate or root, together with a varj"ing number 

 of columnals above it; these columnals are short, but very broad, and in the 

 bourgueticrinoid type of colunni their articular surfaces are usually more nearly 

 circular in outline than are those of any of the other colmnnals except the redupli- 

 cated proximales; they attained a fixed length when the animal was very small, 

 and further increase has been entirely in the direction of acklitional breadth 

 through the process of peripheral accretion so that, with increasing age, they 

 become continually broader and proportionately shorter. Almost imperceptibly 

 these columnals characteristic of the distal region transform into the columnals 

 of the middle region; these latter are more slender, but actually and proportionately 

 markedly longer; they are formed at the period of adolescence, which is the period 

 of maximum growth power. Very gradually these columnals change into the type 

 characteristic of the proximal region; the columnals of the proximal region are 

 shorter than those of the middle region, and any ornamentation or other distinctive 

 feature which the column may possess is upon them greatly accentuated: they 

 mark the passing of the adolescent period of maxinunn growtii power and the 

 assumption of the perfective (as opposed to the purely vegetative) vigor of maturity. 



In order properly to appreciate the column in its relation to the other units of 

 the comatulid whole, and esj)ecially in its relations to the centrodorsal, and to appre- 

 ciate the essential similarity between the columnals, individually and collectivch', 



