the Crinoidea, Cjstidea, a^id Blastoidea. 143 



the ainbulacral orifice. This accords well with an important 

 observation of Professor Tliomson's on the structure of A^ite- 

 don while in the earlier periods of its growth. " The entire 

 body of the Pentacrinoid is," he says, " at first, while yet in- 

 cluded within the pscudembryo 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 system may expand and multiply so as to form a tes- 

 sellated external skeleton to the mature animal, the radial 

 system being entirely absent or represented only in the most 

 rudimentary form " [o]). cit. p. 541). Such is the structure of 

 all the Cystidea. On referring to fig. 2, it will be seen 

 that the whole of the body of Caryocystites testudinanus is 

 covered with polygonal plates, without any trace whatever of 

 a radiated arrangement. The plates are disposed in nine 

 transverse ranges, girding the body like so many rings. This 

 species is (and so are most of the elongated subcylindrical 

 Cystideans) annulated rather than radiated, so far as regards 

 the external integument. The lower range, below the line h, 

 consists of the basals, whilst the upper, above the line ?•, may 

 possibly be radiated. In all the globular or ovate Cystideans 

 with numerous plates, such as Sphceronites, Malocystites, Co- 

 marocystifes^ Amyc/dalocystites^ 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 ar- 

 ranged without any well-defined or constant order. It seems 

 clear, therefore, that the test of the Cystidea belongs mostly 

 to the perisomatic system. 



In Pentremites the three plates which are usually called the 

 basals consist each of two pieces, one placed above the other, 

 and in general closely anchylosed together. The lower pieces 

 have each a re-entering angle in their upper edges, for the 

 reception of the upper pieces which stand upon them. This 

 structure was first pointed out by Mr. Lyon (G-eol. Ky. vol. iii. 

 p. 468), and is not generally admitted, although I believe it 

 certainly does exist. It is said that the lower pieces consist of 

 the upper joint of the column, divided into three by vertical 

 sutures. 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 An- 

 tedon, where at first the bottom of the cup is formed by the 

 basals, but afterwards principally by the first radials. 



The forked plates are usually called " radials^'' but they 

 certainly do not belong to the radial system. If they did, they 

 would represent the first radials of the Crinoidea, and there- 



