270 THE VOYAGE OF H.M.S. CHALLENGER. 



Family Pextacrinid^, d'Orbigny, 1852. 



Calyx small relatively to the stem and arms, composed of five basals and five radials, 

 with under-basals in one genus. The rays divide from one to eight or ten times. The 

 stem bears verticils of cirri at intervals. Two joints are united by syzyg}- at each node, 

 to the upper one of which the cirri are articulated. The internodes are traversed by five 

 ligamentous bundles which are interradially disposed and give rise to a more or less 

 petaloid figure on the joint-faces. No root nor radicular cirri. 



Remarks. — Although definitions of this family have already been given by d'Orbignv, 

 de Loriol, and Zittel, they have been based almost entirely upon palseontological knowledge, 

 and have not therefore given sufficient prominence to the syzygies between certain of the 

 stem-joiats, and to the ligamentous structures which produce the well known petaloid 

 markings on their faces. The regular verticillate arrangement of the cirri along the whole 

 length of the stem is especially characteristic of the Pentacrinidae among the Xeocrinoids, 

 though there are a few Palaeocrinoids, e.g., Belemnocrinus Jlorifer, in which this peculiarity 

 presents itself. But it does not necessarily follow that the nodal joints in the stems of 

 these older forms are the epizygals of syzygies, as is the case in the Pentacrinidse. 



The same may be said of the so-called Encrinus heyrichi, in wliich Picard has described 

 a verticillate arrangement of the cirri on the stem, without mentioning the presence of 

 any syzygies at the nodes. ^ 



In Apiocrinus and Bourgueticnnus the upper part or even the whole of the stem is 

 entirely free from cirri ; and even when they do occur in verticils, it is only by two at a 

 time instead of by fives, as in all the recent Pentacrinidse except the one species Penta- 

 crinus alternicirrus (PI. X5V. ; PL XXVII. figs. 1-3). Further, there is nothing like a 

 syz}"gy between the two joints forming a node in a Bou)yueticrinus-stem, which are 

 articulated to one another in the usual way. 



It is in fact the characters of the stem, much more than those of the calyx, which 

 constitute the special distinctive mark of the Pentacrinidse. For although Extracrinus 

 is known by its under-basals, the composition of the calyx is identical in Millencrinxjis 

 and Pe Iliac rin us, and also in Metacrinus, if we foUow strict morphology and consider 

 the second radials as really arm-joints. 



The calyx of Balanocrinus is unfortunately not yet known. The genus was founded 

 by Agassiz for a fragment in the Basle Museum, which he supposed to be a peculiar form 

 of calyx. But de LorioP has shown that " ce pretendu calice n'est qu'un fragment de 

 tige attaqu^ et deforme par un parasite." Agassiz had, however, referred the stems 

 associated with it to Pentacrinus suhteres; and de Loriol, finding that the stem-joints of 



' Ueber erne neue Crinoiden-Art aus dem MuschelkaU: der Hainleite bei Sondershaufen, Zeitschr. d. deutsch. geoL 

 GeselUck., Jahig. 18S3, p. 201. 

 * Swiss Crinoids, p. 163. 



