Descriptions of JSfor Crinoids. 247 



marks that a careful stud}- of the different species of Glyiitocrinus, 

 found in the Cincinnati Group, has suggested. The species naturally 

 fall into two groups, the extremes being represented by G. decadac- 

 tylus and G. nealU. The former group is characterized by a round 

 column, comparatively few interradials and interaxillaries, by the 

 slight depression of these areas, by want of iinderbasals, and by the 

 greater amount of ornamentation on the plates. 



The latter group is characterized by a pentagonal column, many in- 

 terradials and interaxillaries in deeply sunken areas, by fully developed 

 under basals, and by the comparative want of ornamentation on the 

 plates. Now these two groups are closely united by a series of inter- 

 mediate forms, of which the G. richardsoni is the last and most im- 

 portant link. It agrees with the first group in its round column, want 

 of under basals, and less numerous interradials; and with the second 

 in its depressed areas, numerous interaxillaries, Avant of ornamentation 

 ou the plates, and division of the free arms. As the development of 

 under basals has alwa3's been considered to be of generic importance, 

 and as the G. nealli for that reason, and the greater number and ir- 

 regular arrangement of its interradials and interaxillaries, seems to be 

 as closel}^ allied to Reteocrinus of Billings, as to Glyptocrinus^ it is 

 especially interesting to find the intermediate form in the species here- 

 with described. Mention is made in the description of the interaxil- 

 \a.vj areas, of a series of branches or fixed pinnula given off by the 

 rays. 



It seems clear that the earliest portion of the crinoid to be devel- 

 oped is the lower part, or that represented by the basal and radial 

 rings of plates and the interradials when present; or rather that here 

 was a center of growth, from which the column grew downward, and 

 the bod}- and its appendages upward. It is not difficult to see that 

 the increase of given functions, due to the progressing maturit}' of the 

 animal, may have enlarged the number of organs devoted to special 

 uses, and thus the ambulacral grooves, at first continued onl}- along 

 the inner face of the radials, were diverted into auxiliary^ grooves on. 

 the ventral or inner side of the processes mentioned above. As the 

 bod}' grew upward, these branches graduall}- lost their free character, 

 and became soldered into the general body wall by the interpolation of 

 its plates, while new branches were given off, above these, to take their 

 places, which eventually became fixed in like manner. In this way 

 the conditions of growth were furnished, and the body was gradually 

 built up. 



As the animal approached maturity, the plates forming the sides of 



