196 p. HERBERT CARPENTER. 



echinida the reverse is the case. In the irregularity of the 

 plating the Holothurians resemble the Crinoids, but even in 

 the latter there are traces of a radiate arrangement in the 

 plating of the disc^ which resembles the alternation of the 

 ambulacra and interambulacra in the test of an Urchin. 

 Thus, in Pentacr'inus} there are row^s of small marginal 

 plates at the sides of the ambulacra, and in the interradial 

 and interbrachial areas between them are the perforated 

 anambulacral plates. These marginal plates at the sides of the 

 ambulacra also occur on the disc and arms of Rhizocrinus , 

 and on the arms of Pentacr'inus, Hijocrhms, Batliycrinus, 

 and of the young Antedon, in which they ultimately undergo 

 resorption. Wachsmuth^ has found them also in the arms 

 of Cyathocrinus and of other Palaeozoic Crinoids, in which 

 they are borne upon small quadrangular plates situated on 

 the outside of each groove, and interlock with one another 

 over the middle line of the groove so as apparently to 

 close it completely. Wachsmuth believes that they were 

 movable, and only closed over the furrow Avhen the arms 

 were folded up. Miiller found an outer row of plates sup- 

 porting the delicate marginal plates of the ambulacra of 

 Pentacrinus of the same nature as those described by Wach- 

 smuth in Cyatlwcrimis, and he seems to have called them 

 adamhidacrcd (fig. xiv, cuV), and to have regarded them as 

 homologous with the similarly named plates in the Asterids 

 and Ophiurids.' In this I entirely concur, and I Avould 

 go still further and compare the double row of marginal 

 plates covering the ambulacral grooves (fig. xiv, sup.') to 

 the ordinary superambidac7^al -plates in the test of the Urchins, 

 and in the Ophiurids. 



In most Ophiurids these plates are arranged in a single 

 row, but they are primitively double, as in the young Aste- 

 rids, in which they ultimately become resorbed. In the 

 Urchins this is not the case, and the ambulacral areas consist 

 of two rows of plates, but they differ from the marginal plates 

 of Pentacrinus and most other Crinoids in being perforated 

 by pores, through w^hich the tubular feet reach the exterior. 

 In Cyathocrinus, however, Wachsmuth has found the apices 

 of these marginal plates to be perforated, so that the course 

 of the ambulacra is marked out by double rows of small 

 pores, very much as in the Urchins. 



Another striking resemblance between the elements of the 



' Miiller, " Pentacrinus" loc. cit., p. 49. 

 2 Loc. cit., pp. 120—124. 



=* See Huxley's " Lectures on General Natural History," ' Medical 

 Times and Gazette/ Dec. 13, 1856, p. 587. 



