93 
dispels this impression, for, in addition to the scnlptnre being concentric to the 
margins of the plate, as a whole, and not to three parts individually, the 
planeness of the plate is opposed to the convexity of the ordinary dorsal cup. 
The first or bottom plate of the ventral tube (PL XXII, Pigs. 10 and 
11) reposing on the top of the azygous plate, and wedged in between the 
posterior radial and anal plates is oblong in shape. Its position thus brings it 
within the ring of radials. 
TribracMocrinus Clarkei, not only differs from T. corruejatus^ Patte, 
in size, sculpture of the plates, and its protuberant posterior end, but also in a 
more important structural point, the much greater ventral extension of the 
anal plate in tbe former species ; that is to say, this plate is longer in 
T. Clarkei than it is in T. corriKjatus, and the second anal correspondingly 
shorter. Prom T. ornatus, which resembles it in possessing an asymmetrical 
calyx, the present species is distinguished by the absence of the cristiform 
centre and radiating ridges on the various plates. Prom T. granulatus, by size, 
proportions of the calyx, and the symmetrical outline of the latter. Lastly, 
Mr. P. M. Johnston has deseribed,^ in words mueh too brief to be of speeific 
value, a Crinoid from the Pachydomus beds of Darlington, Maria Island, 
Tasmania, as Tribrachiocrinus tasnumicus. Prom his remarks I take it to he 
no other than our T. Clarkei ; in fact, Mr. Johnston appears to have been very 
doubtful about its specific value, for he remarks : — “ The above species comes 
very close to, and perhaps may not be specifically distinct from, the smaller 
form with large perforation in tripartite pelvis described by Prof. M‘Coy as 
T. Clarkei.” The length, three inches, is quite equalled by our PI. XIV., Pig. 
3. The only peculiarity about Mr. Johnston’s fossil seems to be the absence 
of the columnnar perforation in the base of the calyx. The plates shown 
in Mr. Johnston’s figures, besides the infra-basals, are the left posterior, 
heptagonal, and pentagonal basals. 
Another point is Avorthy of consideration. I was at first inclined to 
believe that two sj)ecies had been included within the illustrations of T. Clarkei 
now given. Por instance, PI. XIII, Pig. 2-J, represent one, the true species, 
corresponding with M‘Coy and De Koninck’s figures, Avhilst a larger form, 
illustrated by PI. XIV, Pig. 3, and PI. XVII, Pig. 2-1, differed from the 
preeeding in its larger size. As, however, I am not able to detect any 
morphological difference, it appears better to alloAV them to remain under one 
‘ Proc. R. Soc. Tas. for 1886 [1887], p. 233, PI. (central figure). 
