&ENEEA AISTD GEOITPS OP THE ECHIJSTOIDBA. 299 



" elongate f when the Apical system has the posterior pair of Badial 

 plates not in contact with it, but jAsiced far posteriorly and separated from 

 2^; by the junction of inter radial plates along the dorsum, it is said to be 

 ^^ disjunct or disconnected^ 



The Basal plates project into, or are in contact with, interradia, and the 

 Badial plates surmount amhulwcra. In a great many genera " the Madre- 

 porite" is in the right anterior basal plate, which then is the largest; the 

 left anterior basal plate is at that side of the right anterior, and the postero- 

 lateral basal plates are immediately posterior to the anterior pair, except in 

 Elongate apical systems, where, as has been noticed, the antero-lateral 

 Eadial plates intervene. The posterior Basal plate is in the antero-posterior 

 median line of the dorsum of the test and is in contact on either side and 

 to the front with the postero-lateral basal plates ; or it may be absent. 



The anterior or odd Badial plate is in the median line anteriorly, and is 

 in the angle made by the antero-lateral basal plates ; the postero-lateral 

 Badial plates are on either side of the posterior Basal plate. When there 

 is no posterior Basal plate, \h.% posterior pair of Badial plates may come in 

 contact and close the system posteriorly, or they may be separated by the 

 Madreporite extending backwards. The anterior Badial plate is No. III., 

 the left anterior Badial plate is No. IV., the left posterior is No. V., the 

 right posterior Badial plate is No. I., and the right anterior plate, usually 

 with the madreporite, is No. II. The ambulacra corresponding to the plates 

 are similarly numbered. The right anterior basal plate is No. 2, the 

 opposite or left anterior Basal plate is No. 3, the left posterior plate is 

 No. 4, the posterior Basal is No. 5, and the right posterior Basal plate is 

 No. 1. The interradia in relation with the Basal plates are similarly 

 numbered. (See Loven, Etudes.) 



The Madrep)orite. — A cribriform structure consisting of canals, tubular in shape, 

 variable in number, with reticulate carbonate of lime between them and 

 forming their walls, opening at the surface of the test, the orifice or 

 orifices being minute and rather close, leading into the test to a canal 

 which is in communication with the water-system and a renal organ. It 

 has no especial plate and the tubules of the body may be restricted to the 

 area of the right anterior Basal plate, or may extend and perforate other 

 Basal plates and even some interradial plates. The Madreporite may 

 exist so as to separate some of the basal plates in the Exocyclica, to form 

 much of the centre of the system, and it may pass backwards separating 

 the posterior Basal plates and even the posterior Radial plates, and may 

 abut against the posterior interradium. In some Clypeastroida there may 

 be but one pore in the Madreporite, or two or three situated along a narrow 

 groove ; in the majority the body may be central. 



When a Madreporite in the Exocyclica is bounded posteriorly by the 

 posterior pair of Basal plates, Nos. 4 and 1, the apical system is Ethmo- 

 phract ; but when the posterior Basal plates and the Postero-lateral 

 Radial plates are pushed on one side by the Madreporite, the arrangement 

 ifi Ethmolysian. (See Loven, Etudes.) 



In the Exocychca the periproct is in tlie median line of the posterior 

 ambulacrum, beyond the apical system ; it may be circular, elliptical, 



