Epistroma (Loven, Ech. desc. by Linnseus). yi, ^ 4^- i -U 



298 PEOF. p. M. Duncan's eevision op the 



scrohicular area : its outer limit, more or less circular, is the scrobicular 

 circle, and may or may not be perfect; if not, the scrobicule of one 

 tubercle will be found to merge into those of the tubercles placed dorsally 

 and actinally to it, and is " continuous!^ Secondary tubercles may or may 

 not hare scrobicules. Yery small tubercles incomplete in their division 

 into parts are oniliaries. Granules are more or less nodular projections of 

 the test, and may be large and distant or very numerous and like shagreen, 

 and all intermediate sizes are seen. The tubercles carry spines. The 

 miliaries and granules may carry small spines or pedicellarise. 



Loven has lately pointed out that much of the ornamentation of certain 

 genera, such as ridges, all moniliform, pedunculate, and large granules, 

 huear or vermiform elevations, and groovings, is due to the calcification of 

 a membrane placed in early life upon the plates. This ornamentation is 



J"' 



Borso-central or Apical System, — This is abactinal or dorsal, and consists of 

 Basal plates, Eadial plates united by sutures or fused or separated by soft 

 tissue (formerly termed Genital and Ocular), the madreporite, and of 

 the periproctal plates in some, and sometimes of intercalated plates. In 

 the Endocyclica the periproctal ring, composed of basal and sometimes 

 of radial plates, surrounds the membrane and plates of the periproct 

 and the anal opening, with or without anal plates. The genital glands, 

 within the test, open externally by ducts, and one or more of them may 

 perforate a Basal plate, or the perforation may be in the interradium 

 beyond the dorso-central system. All or some of the basal plates may 

 be thus perforated. Each Eadial plate has a canal with a single or 

 double orifice, and in the Paliseechinoidea more than one canal may exist ; 

 the position of the opening differs, but it appears to refer to the primi- 

 tive large tentacle, and not to an ocular organ. The Eadial plates, 

 five in number, may either be placed within the angles formed by the 

 actinally projecting Basal plates, and are then external, or one or more may 

 project between the Basal plates and separate them. In the Endocyclica 

 if the Radial plates touch the margin of the periproct they "enter the 

 ring." The Basal plates are either five or four in number, and when the 

 latter number prevails it is due to tlie absence or incomplete development 

 of the posterior Basal plate. The Periproct is within the Borso-central 

 system in the Endocyclica, and there are plates covering it more or less. 

 The plates may be few and symmetrical and triangular, or numerous and 

 more or less ovoid, circular, or irregular in outline, and placed concentri- 

 cally, the largest anteriorly. A sur-anal plate may exist and is large, and 

 in front of the anal orifice or in front and to the left side ; rarely a some- 

 what continuous pavement of plates occurs. The plates carry spinules, 

 tubercles, granules, pedicellariEe. The shape of the periproct varies: it 

 may be circular, elliptical, or pyriform ; it may have its long axis along 

 the median line of the dorsum or obliquely to it in the Endocyclica, 

 When the test is Exocyclic, the Basal plates may be in contact at their 

 sides, forming a " compact" system ; when some of the Eadial plates, i. e. 

 the antero-lateral pair, separate the Basal plates, and unite along the 

 median line, pushing the posterior basal plates backwards, the system is 



