IN THE MORPHOLOGY OP THE CTSTIDEA. 21 



argument in favour of this view. But the number of the ambu- 

 lacra in a Crinoid or Cystid is not a satisfactory test for deter- 

 mining the primary symmetry of the type. A similar condition 

 to that of the hexamerous Caryocrinus with three primary 

 ambulacra is presented by the ten-rayed Promachocrinus. the 

 disk of which has only five primary ambulacra. In Actinometra, 

 which has a pentamerous calyx, the number of ambulacra joining 

 the peristome may vary from three to ten, and those of the 

 posterior rays do not by any means coincide in position with the 

 radial plates of the dorsal side. In Ecliinosplicera aurantium 

 there may be two, three, or four ambulacra *. Pseudocrinus 

 has but two, though its dorsal cup is pentamerous, and the latter 

 is also the case in SpJKsrooi/stis, Apiocystis, or Lepadocrinus, which 

 have but four primary ambulacra. The hexamerous Semicosmites 

 has a triradiate peristomial area, while Juglandocrimcs, also hexa- 

 merous, has three pairs of ambulaeral openings ; the symmetry of 

 the ambulacra thus indicated is in each case the same as in 

 Caryocrinus (figs. I.-III.). "Wachsmuth and Springer have pointed 

 out how in the latter genus the three primary ambulacra coincide 

 in position with three of the six summit-plates, the anterior and 

 the two postero-lateral ones. I have indicated the course of these 

 subtegminal ambulacra in a figure of the summit, which shows the 

 interradial position of the six orals (o),as determined by the sym- 

 metry of the dorsal cup (fig. II.). Since there are only three 

 primary ambulacra which supply three pairs of radials (11-12, 

 14-15, 16-17), it is obvious that they would naturally occu^Dy 

 interradial positions and so come to lie beneath three of the orals, 

 which were not movable like those of Neocrinoids, but formed 

 part of a rigid tegmen. This is well shown in fig. II. (See 

 Postscript, infra, p. 51.) 



In Juglcmdocrinus the ambulacra are subtegminal, but Von 

 Koenen's description f shows clearly that there must be three 

 primary trunks, each opening externally by two pores on the 

 edges of the plates which he marks m (fig. III.). Von Koenen 

 says in reference to these openings : — 



'' Sehr eigenthiimlich sincl bei unserer Art die sechs paarig angeordrieten 

 Loclier im Seheitel. Falls sie niclit, wie bei den Palffiocrinoiden, auf Arm- 

 Ausatze zu deuten sind, was bei ihrer Lage wenig wahrseheinlich ist, wilrde eine 



* See Volboi'th, "Ueber die russischen Sphaeroniten," Verh. min. ues. St. 

 Petersburg, 1845-46, p. 18 (of separate copy), and Taf. ix. figs. 6-8, 

 i" Loc. cit. p. 253. 



