MOEPHOLOGICAL PAET. 41 



stem is enlarged somewhat as in Apiocrimis, except that the joints of the en- 

 larged part in the Ichthyocrinid^ are very short, and increase but very 

 slightly in length downward. These plates, which have no internodals inter- 

 posed, extend to the full length of the inflated part, and their number varies 

 in different species from about twenty to fifty, but is constant, or nearly so, 

 in the same species. They are followed distally by a large prominent 

 joint, from which the internodes begin. The latter are formed in the usual 

 way, larger and smaller joints following each other at intervals, and the 

 nodal joints are rather prominent. 



The stem of a Platycrinus has generally no internodes, and all young joints 

 were introduced next to the basals. The joints are elhptic, and the apposed " 

 faces of the joints throughout this genus are provided with articular ridges, 

 which follow their long diameters. A similar structure occurs in the recent 

 EMzocrimis and Bathi/crimis, and both of them are apparently destitute of 

 internodals. The Silurian Marsupiocrinus, however, with a circular stem, 

 which is otherwise most closely allied to Platycrimis, always has well defined 

 internodes ; and this forms perhaps the best distinction between the two 

 genera. 



The absence of internodals is not confined to specimens with elliptic 

 stems, or to those with articular ridges. They are wanting also in Mespi- 

 locriniis with a round stem, and in which the joints rapidly attain a length of 

 from three to four times their diameter (Plate IL, Fig. 3). In Rhodocrinus 

 there is, so far as observed, but a single ossicle to each internode, and 

 throughout the stem a larger plate alternates with a smaller one. 



In a few Palaeozoic Crinoids, the whole stem is divided longitudinally, its 

 joints being either quinque- or tri-partite. The former is the case in Ohio- 

 criniis^ Edenoenmis, Bari/crinus, Anomalocrmus, and probably others ; while a 

 tri-partite stem has been observed only in Heferocrinus. The stem segments 

 alternate with the proximal plates of the calyx ; i. e.^ they are interradial 

 in dicyclic, and radial in monocylic Crinoids. 



Most Crinoids are provided with cirri, which are given off from the nodal 

 joints at intervals, either throughout the whole length of the stem, or only at 

 its distal end. The former is more generally the case among the later Cri- 

 noids, while in the majority of Palaeozoic forms the cirri are restricted to the 

 lower part. In Neocrinoids they are more regularly distributed, and occur 

 in whorls ; in Pal^ocrinoids they are generally arranged singly, and at irregu- 

 lar intervals. The Pentacrinidae have five cirri to each nodal joint, which 



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