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256 THE VOYAGE OF H.M.S. CHALLENGER. 
of Bourgueticrinus (ellipticus and e@qualis) the uppermost part of the stem is formed of 
joints of an altogether different character from those which are to be found lower down. 
The latter are wider than high, with articular faces of a pointed oval shape, the two 
diameters of the oval being very unequal in length. Towards the upper part of the stem 
this inequality disappears, and the joints become more discoidal, though never thin and 
lamellar, as in the highest part of the stem of Rhizocrinus and Bathycrinus (Pl. VIL 
figs. 2,11; Pl. VIlla. fig. 1; Pl. IX. figs. 1-3; Pl. X. fig. 2). Above these discoidal 
joints, however, the stem gradually enlarges, and from two to four of the uppermost 
joints on which the calyx rests are of great relative size, both height and diameter being 
considerable. Sometimes the top joint is highest and sometimes the one below it; but 
at any rate one or more of these large upper joints remain united with the calyx to form 
the so-called summit. The upper stem-joints of Apiocrinus are not as a rule much 
higher than those below them; but the diameter often increases considerably from a 
point in the stem a little way below the calyx, so that a tolerably large number of joints 
enters into the composition of the summit. In species like Apiocrinus crassus, 
Apiocrinus magnificus, and Apiocrinus murchisonianus, however, there is scarcely any 
enlargement of the stem below the calyx, the uppermost joints, except the highest one 
on which the basals rest, being but little if at all larger than those below them. The 
same variations appear in Millericrinus. Millericrinus nodotianus has high upper joints, 
while those of Millericrinus simplex are thin and discoidal externally ; though the upper- 
most joint has a large synosteal surface for the reception of the basals which rest upon it. 
It seems to me very probable, therefore, that the existence of similar variations will 
have to be admitted in Bowrgueticrinus; though on the other hand a revision of the 
genus may result in the transfer of all the species without enlarged upper stem-joints to 
Rhizocrinus or Mesocrinus, especially if the articular faces of their radials are well 
developed and not reduced to a minimum as in Bourgueticrinus equalis. But I am 
quite prepared to have to abandon Mesocrinus as a distinct generic type, and to modify 
the descriptions of Bouwrgueticrinus which are given by paleontologists so as to include 
in this genus the two species Mesocrinus fishert and Mesocrinus suedicus, on which the 
genus Mesocrinus was based. 
Even then, however, Apiocrinus and Bourgueticrinus would differ from Rhizocrinus 
and Bathycrinus in the character of the upper stem-joimt. In the two genera last 
mentioned, as in Pentacrinus, this joint is the youngest and smallest of the whole stem, 
being merely a delicate film of calcareous reticulation which is received into and concealed 
by the curved under surface of the basals. In Apioerinus, however, and in Bourgueti- 
crinus this upper joint, though perhaps small externally, is large internally and supports 
the basals in five large fossze on its surface. 
As it closes the calyx below and really belongs thereto, de Loriol’ has called it the 
1 Swiss Crinoids, p. 6; Paléont. Frang., loc. cit., p. 19. 
