ME. P. II. CAEPENTEE ON THE GENUS ACTINOMETEA. 21 



simple, as in Ant. rosacea, or divide more or less frequently into secondary, tertiary, 

 &c. arms, as in Act. multiradiata and in Pentacrinus; and every segment, like the 

 radial axillary, preceding- a bifurcation, was called by Muller a "bracbial axillary." 



In some of the Tessellate Crinoids, however, the arms do not become free at the radial 

 axillary, but " der Kelch setzt sich noch weiter fort ; die lladien zerfallen dann in zwei 

 Distichalradien mit radialia distichalia, die jedes mit einem distichale axillare endeu, 

 wie bei Actinocrinus moniliformis und Eucalyptocrinus " \ In this case the distichal radii 

 represent the primary arms of Comatula and Pentacrinus, though Muller never used 

 the name " distichals " in his descriptions of the species of Comatula; for, as in the 

 Tessellata the segments composing two adjacent distichal radii are united laterally with 

 one another by intermediate plates, he regarded them as forming a part of the calyx, 

 and considered the arms of this group as starting from the distichal axillary, and not 

 from the radial axillary, as in the Articulate Crinoids. 



The two primary arms, or distichal radii, borne upon a single radial axillary, were 

 called by Muller a " distichium ; " and the interval between two successive distichia 

 dorsally between the calcareous segments, or ventrally between the corresponding 

 grooves on the disk, was spoken of by him as " mfcrpalmar,' 5 while the interval between 

 the two primary arms or distichal radii borne by the same radial axillary, or, as Muller 

 called it, "die Kluft eines Distichiums," was " interbrachial " or " intrapalmar." 



The words " interambulacral," " interradial," and " intertentacular," have been also 

 used by Muller and others to designate the interpalmar areas on the disk of Comatula. 

 Either of these is preferable to " interpalmar," for reasons which will presently appear, 

 though " intertentacular " is not universally applicable, as in certain Actinometrce the 

 posterior ambulacral grooves bounding the large area in which the anal tube is situated 

 are not provided with tentacles at their sides. 



The term "interbrachial" is decidedly preferable to "intrapalmar," which was used 

 by Muller to designate the small areas on the disk, bounded by the two branches of 

 each of the five primary groove-trunks. " Intrapalmar " does not convey any clear idea 

 of the relation of these areas to the divisions of the skeleton, while "interbrachial" dis- 

 tinctly indicates that they correspond to the intervals between the two primary arms 

 borne by every axillary radial. 



Reenter 2 adopted Muller' s nomenclature for the fossil Crinoids, and, like him, con- 

 sidered that the distichal radii, when present and united laterally to one another, formed 

 a part of the calyx ; so that the arms were regarded by him as commencing from the 

 axillary distichals, while he distinguished their different divisions simply as rami of the 

 first, second, and third order. De Koninck and Le Hon 3 , however, regarded the arms 

 as commencing from the first bifurcation, i. e. from the axillary radial, whether they 

 become free at once or whether they remain united with the calyx for a longer or shorter 

 distance. Nevertheless they distinguished the arm-segments by different names in the 

 two cases, using the expression " pieces brachiales " for the distichals of Muller, i. e. for 

 those segments which are immovably united with the calyx, while they gave the name 

 " articles brachiaux " to the movable segments, the brachials of Muller. 



1 Baudes Pentacrinus, p. 31. " Lethoea geognostica, Band i. Theil 2, pp. 210, 215. 



3 Recherches sur les Crinoides du Terrain Carbonifere de la Belgique, (Bruxelles, 1854) p. 69. 



