LFXAN0CRIN1DAE 191 



resulting in a lack of stability in some of the more important generic, and even larger, 

 characters. It has somewhat the features of a degraded type ; the crown is, small in propor- 

 tion to the stem, and instead of being erect often hangs downward, the upper part of the stem 

 being curved or coiled so as to permit this. This is not a mere accidental position due to 

 contraction at death, but while not accompanied by such an extreme modification as we find 

 in Calceocrinus, it nevertheless represents a definite structure. The first few proximal 

 columnals are cuneiform, and the adaptation of their shape to the curvature of the stem is 

 perfectly shown in a side view of the coil (PI. V, figs. 13a, b). With such a construction it 

 does not seem possible that the crown could at any time have had an erect position. This, 

 feature is very prominent in M. konincki, but not so much so in M. foroesianus. 



Along with this aberrant stem structure, there appears in this genus a very peculiar 

 asymmetry in the disposition of the rays, by which they are bent to the right, so that instead 

 of standing parallel to the vertical axis of the crown they tend to twist around it from left to 

 right. As in the case of the stem, this feature is accompanied by a structural modification 

 whereby the radials and lower brachials lose their symmetry of form ; their lateral margins 

 are of unequal lengths, their angles quite irregular, and their size unequal. These two 

 characters do not always go together. In M. konincki, which seems to represent the extreme 

 of specialization in this genus, both are conspicuous, and are also accompanied by a further 

 striking modification of the stem in the development of spindle-shaped columnals of extraordi- 

 nary length, some of them. being ten times as long as their terminal diameter. M. for- 

 besianus has the twisted and asymmetric rays, with a stem much less coiled and in other ways 

 entirely different. M. chapmani has the drooping crown and partially coiled stem, with elon- 

 gate but not spindle-shaped columnals ; but it has not the asymmetry and twist of the rays. 



With such instability of the characters that should distinguish genera, it is not surprising 

 to find a further instability in one of the most strongly fixed characteristics of the whole 

 group, viz., the three infrabasals, which prevail otherwise throughout the Flexibilia without 

 exception save in the case of Nipterocrinns where they are coalesced. Here the infrabasals 

 occasionally, and within the same extreme species, M. konincki, tend to revert to the funda- 

 mental five. 



In addition to these erratic features there is also a tendency to variation in regard to the 

 lateral contact of the rays. In at least one species, M. thiemei, the first primibrach does not 

 fill the distal face of the radial, but leaves short shoulders on either side, while the arms are 

 rounded, leaving small open spaces between them. Whether these spaces were occupied by 

 small plates cannot be ascertained, but there is room for thinking this possible. Such a modi- 

 fication leads in the direction of a form like Pycnosaccns, and a species in which it was cer- 

 tainly developed would fill a place in the series as the Carboniferous representative of that 

 genus freed from its radianal. 



Mespilocrinus is characterized by an extraordinary enlargement of the posterior basal, 

 which rises almost to the level of the distal face of the radials, filling part of the space occu- 

 pied by the radianal and anal in Lecanocrinus. This feature is constant in all the species with 

 the possible exception of M. bordeni, in which the anal side is unknown ; it exhibits little 

 variation, so that it is of no service in the separation of species. The genus is also remark- 

 able for the frequent presence of only two secundibrachs. In one species, M. konincki, this 

 is almost constant ; in M . forbesianus the number varies from two to three ; while in M. bor- 

 deni there are as many as seven — thus showing a rather unusual range of variation even in this 

 somewhat variable character. The smaller number is also found in the latest species of 

 Taxocrinns, T. whitfieldi. The surface in all species known is smooth. 



Schultze, not understanding at that time the significance of the radianal, ranked this 

 genus as a synonym under Lecanocrinus. 



Miller and Gurley described a specimen of M. konincki from the Choteau limestone of 

 Missouri (equivalent in part to the Lower Burlington) under the name of Cyathocrinits 



