226 E. Billings on the structure of Crinoidea, Cystidea, ete. 
of Professor Thomson’s on the structure of Antedon, while in 
the earlier periods of its growth. “The entire body of the Pen- 
tacrinoid is,” he says, “ at first, while yet included within the 
pseudembryo and during its earliest fixed stage, surrounded 
and enclosed by plates of the perisomatic system alone, and it is 
quite conceivable that plates belonging to this s stem may ex 
pand and multiply so as to form a tessellated external skeleton 
to the mature animal, the radial system being entirely ener 
or represented only in the most rudimentary form.” (Op. cit., p. 
541). Such is the structure of all of the Cystidea. On eats 
ring to fig. 2, it will be seen that the whole of the body of Cary- 
ocystites testudinarius, ' is covered with polygonal plates, without 
any trace whatever of a radiated arrangement. The plates are 
disposed in nine transverse ranges, garding the body like so gh 
rings. This species is, (and so are most of the elongated su 
cylindrical Cystideans) annulated ether than radiated, so far as 
regards the external integument. The lower range, below the 
line, 6, consists of the basals, whilst the upper, above the line, 7, 
may, possibly, be radiated. In all the globular or ovate Cysti- 
deans, with numerous plates, such as Spenonites, Malocystites, 
Comarocystites, Amygdalocystites, and others, the shell is neither 
annulated nor radiated, but composed of an indefinite number 
of plates, increasing with the age of the individual, and arranged 
without any well defined or ‘constant order. It seems clear, 
therefore, that the test of the Cystidea belongs mostly to the 
perisomatic system 
Tn Pentremites the three plates which are usually called the 
basals, consist each of two pieces, one placed above the other, 
gue in general, closely anchyclosed together. The lower pieces 
ntering angle, in their u tnd edges, for the re- 
1 i upon them. oii, 
To me they appear to be calycine plates. It is true that 
they do not form the bottom of the visceral cavity, but this 
may be due to the growth inward of the lower edges of those 
of the upper series. Something like this occurs in Antedon, 
where, at first, the bottom of the cup is formed by the basals, 
but afterwards rincipally by the first radials. 
The forked plates are usually called “ Radials,” but they cer- 
tainly do not belong to the radial system. If they did, they 
would represent the first radials of the Crinoidea, and therefore 
they should support the bases of the ambulacra. A little ¢ 
sideration will, however, enable any one to perceive shit tr in 
Pentremites the bases of the ambulacra, are situated in the apex 
