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



of Bourgueticrinus (ellipticus and cequalis) the uppermost part of the stem is formed of 

 joints of an altogether different character from those which are to be found lower do-wii. 

 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 (PI. VII. 

 figs. 2, 1 1 ; PI. Villa, fig. 1 ; PL IX. figs. 1-3 ; PL X. fig. 2). Above these cUscoidal 

 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, 

 Ainocrinus magnijicus, and Ajnocrimis onurchisonianus, 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 nodotianush.as high upper joints, 

 while those of Millericrinus simjilex 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 wiU 

 have to be admitted in Bourgueticrinus ; 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 cequalis. But I am 

 quite prepared to have to abandon Mesocrinus as a distinct generic type, and to modify 

 the descriptions of Bourgueticrinus which are given by palaeontologists so as to include 

 in this genus the two species Mesocrinus Jisheri and Mesocrinus suedicus, on which the 

 genus Mesocrinus was based. 



Even then, however, Apiocrinus and Bourgueticrinus would difi'er from Rhizocrinus 

 and Bathycrinus in the character of the upper stem-joint. 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 Apiocrinus, however, and in Bourgueti- 

 crinus this upper joint, though perhaps smaU externally, is large internally and supports 

 the basals in five large fossse on its surface. 



As it closes the calyx below and really belongs thereto, de Loriol ^ has called it the 



1 Swiss Crinoids, p. 6; Paleont. Fran?., loc. cit., p. 19. 



