27,2 SMITHSONIAN INSTITUTION 



right (which may represent the radianal of the earlier form) is above the radial instead of 

 below it. 



The two earliest species of this type come from the Upper Burlington limestone, no 

 example of it being known from the rich and abundant crinoidal deposits of the Lower 

 Burlington or Kinderhook or their equivalents. One of these Upper Burlington species 

 (the only one in the genus) has only two primibrachs — the celebrated and splendid For- 

 bes io cri nus agassizi, always considered the rarest prize of the Burlington collectors, and one 

 of the most magnificent known species of the Crinoidea Flexibilia. It is essentially a 

 Sagenocrlnus minus the radianal, with an elongate posterior basal and a proximal enlarge- 

 ment of the stem. What the line of succession was through the Devonian we do not know ; 

 the geological record is silent as to that ; but it is evident that the increase in the number of 

 primibrachs which went on in this family from the Silurian to the Carboniferous parallel 

 with the Taxocrinidae was practically accomplished in this genus as we first find it. For 

 alongside of F. agassizi in the same beds there is a species persistently diffe'ring from it in 

 size, in which the three primibrachs characterizing the final stage of the genus are constant ; 

 and throughout the abundant Keokuk species, up to the conspicuous forms which mark its 

 extinction in the Warsaw, there is no departure from this character. Therefore, whether we 

 consider F. agassizi as a case of retarded development or of retrogression, it is clearly such 

 an exception as may be recognized without impairing the generic unity. 



This genus affords the material for a very interesting study of the articulations and 

 unions by which the plates of the calyx are connected, and by which the flexible character 

 which marks the whole group is beautifully illustrated. I have for many years past secured 

 all the loose plates of Flexible crinoids that have been found by the collectors at Tournai, 

 where the fossils are often much disintegrated, falling to pieces when dislodged from the 

 soft matrix ; while unfavorable for the production of complete specimens, this results in fur- 

 nishing many isolated plates with the surfaces perfectly exposed by weathering. We thus 

 obtain material for the study of articulating surfaces far better than is found in most Amer- 

 ican localities, where specimens intact are much more abundant. Nearly all of these plates 

 belong to species of this genus, as is shown by the character of the lateral faces ; a very few 

 are referable to Taxocrinus, of which one complete specimen has been found. 



In the Tournai species of this genus there are five kinds of articulations or unions between 

 the plates, which are fully illustrated by the various figures on Plate XXIV : 



i. Between infrabasals and the proximal or upper columnal. This is by short ligaments 

 and finely striated contact areas, like that between any two-stem ossicles. I have not a speci- 

 men to illustrate it in this genus, but from the appearance of the stem and attached infra- 

 basals in Plate XXIII, figure 3, I assume it to be the same as in many other Sagenocrinidae 

 and Taxocrinidae, being a close suture like a stem syzygy, much firmer than those between 

 succeeding plates, thus causing the infrabasals to adhere usually to the stem when the 

 specimen is disintegrated. 



2. Between infrabasals and basals ; basals and .radials ; radials with each other and with 

 interbrachials ; interbrachials among themselves and with primibrachs up to the angle of 

 IBr 2 . This is a non-muscular union by connective tissue or ligament ; there is a fine border 

 of fine interlocking crenulation, inside of which is a deeply sunken fossa occupying a large 

 part of each face of the plate, evidently for the lodgment of very large and powerful liga- 

 ments or elastic tissue. On these faces only a small portion of the apposed surfaces of the 

 plates is in close contact, being confined to that marked by the crenulated border, while the 

 main connection is by means of a large mass of soft parts, affording the most perfect example 

 of that flexible character of the calyx walls which in various degrees — probably in some 

 forms almost lost — characterizes the whole group (PI. XXIV, figs, la, b, 2b, 3, 5a, b, yc, 

 8c, 28c). It will be noted that this is also the mode of union between the posterior basal and 

 the succeeding anal plates (Pis. XXIII, fig. 3 ; and XXIV, fig. 4). 



