REPORT ON THE CRINOIDEA. 25 



While the general characters of the stem are identical in RMzocrinus and Batliy- 

 crinus, there is a good deal of variation in its details, and especially in the mode of 

 growth. 



In all cases the new joints are added at the top of the stem, immediately beneath 

 the cup ; but the rate at which they increase in length is very diflferent in the diflferent 

 species. It appears from Sars's figures,^ and from my own observations, that the pro- 

 duction of new joints in the stem of RMzocrinus lofoteiisis is slow compared to their 

 subsequent increase in length. For there are very rarely more than three joints beneath 

 the cup which arc wider than high, and even these have an appreciable thickness 

 (PI. IX. figs. 1, 2 ; PI. X. fig. 1). Sars remarks of the uppermost one that it is 

 "annulaire et a 2 ou 3 et meme souvent 5 ou 6 fois plus de largeur que de hauteur;" 

 while there are usually not more than eight cylindrical joints beneath it. Below this 

 limit the joints have the well-known dice-box shape, with the characteristic terminal 

 faces, the peculiarities of which begin to appear very few joints below the cup. 



Most individuals of Rhizocrinus rawsoni seem to be generally similar to RMzocrinus 

 lofotensis in these characters (PI. IX. fig. 3 ; PI. LIII. figs. 7, 8). But in one example 

 I found five joints beneath the cup which were wider than high. The second and third 

 are mere circular disks with perfectly plain faces like those of the fourth (PL X. fig. 10) ; 

 and the faces of the newly formed joints of Rhizocrinus lofotensis which are figured by Sars ^ 

 are of the same nature. But the uppermost joint of all is of a difierent character 

 altogether (PL X. fig. 9). It has a pentagonal outline, and its surface, which rises 

 gradually from the circumference towards the centre, is divided by five radiating ridges 

 into an equal number of trapezoidal fossae that receive the lower ends of the elongated 

 basals (PL X. figs. 3, 5). Here, therefore, we find the top stem-joint presenting the 

 same characters that it does in Apiocrinm ^ and Millerioinus,^ and entering to some 

 extent into the composition of the cup, while the new joints are probably intercalated 

 below it. Quenstedt^ speaks of this uppermost stem-joint in the Apiocrinida^ indif- 

 ferently as "Endstiick, Endsaulengiied, or Fiinfrippenglied." De LorioP has named it 

 " article basal ;" while Zittel ' speaks of it as the " Centro-dorsal," and remarks " Dasselbe 

 scheint, wie aus der Andeutung von Nahten hervorgeht, aus 5 ursprunghch getrennten 

 Stucken entstanden zu sein und entspricht wahrscheinlich den 5 Infrabasalplattchen 

 bei Encrinus." It is perhaps a little inexpedient to employ the term " centro-dorsal " for 

 a joint which bears no cirri, as its similarly named homologue does in the Comatulae. 



1 Memoires pour servir h, la connaissance des Crinoides vivants. 1. Du Bhizocrinm lofotensis, tab. L, ii. 



2 Op. dt., tab. ii., figs. 20-22. 



3 D'Orbignj, Histoire Katurelle, generale et particuli^re des Crinoides vivans et fossiles., pL ii. fig. 3, pi. iii. 

 fig. 4, pi. V. fig. 4. 



^ Ibid., pi. xiv. figs. 15, 23, 24 ; and Quart. Jouni. Geol. Soc, vol. xxxviiL p. 33, pi. l. 



5 Encriniden, pp. 314, 315. 



* Swiss Crinoids, p. 4 ; Paleont. Frang., loc. cit., p. 19. 



' Handbuch der Pakeontologie. Palseozoologie, Bd. i., pp. 388-390. 



(ZOOL. CHALL. EXP. PART XXXII. 1884.) •" * 



