ECHINOIDEA. 557 



addition to the two large pores connected by a furrow {yoked pores) at the 

 outer edge. And the remaining plates of the ambulacra may possess 

 similar rows which may extend on to the interambulacral plates forming 

 pore-areae. In the family Scutellidae these fine pores are arranged on the 

 actinal surface in lines -pore-fasciae sometimes branched. The pore plates 

 in Cidaridae, Clypeastroidea, and Petalosticha retain their regular succession. 

 In the two last named the dorsal sections of the ambulacra gradually dilate 

 from the apex and then contract more or less completely near the margin 

 of the test forming petala, arranged in a rosette. In Petalosticha however 

 the anterior ambulacrum is often unlike the rest, not much expanded, and 

 its pores small or obliterated. The remaining sections of the ambulacra 

 have plates so much expanded laterally in Clypeastroidea that the peri- 

 stomial ends of the interambulacra are often constricted or excluded from 

 the peristomial margin. In the Cassidulidae the peristomial ends of the 

 ambulacra dilate into petala or phyllodes, forming a figure known as 

 floscella. In the Ectobranchiate Desmosticha on the contrary the pore- 

 plates at a little distance from the apex lose their typical arrangement. 

 Certain plates (half plates) remain small in size and fail to reach the central 

 line of the ambulacrum : others increase in size, and there is a fusion of 

 three or more primary plates to form secondary plates. Near the peristome 

 the secondary plates themselves may fuse. As to the interambulacral plates, 

 they become much expanded near the peristome of Spatangidae, espe- 

 cially in the posterior meridian, where the single plate ending the series is 

 expanded laterally, forming the labrum. It is preceded by two broad 

 elongated plates, the sterna, and these by the episterna. The whole struc- 

 ture constitutes a raised plastron. The interambulacral plates in the 

 typical Clypeastroidea and Petalosticha are irregular in shape 1 . 



The test as a rule retains a regular outline. In the Scutellidae, how- 

 ever, notches often correspond to the ambulacra and to the posterior inter- 

 ambulacrum at the margin of the flattened disc. They may be converted 

 into foramina during growth, and are sometimes very numerous in the 

 region of the bivium. 



The peristome is membranous. It is covered by plates continuous 

 with the ambulacra and interambulacra respectively, and set free from the 

 corona in the Cidaridae. In ectobranchiate Desmosticha there are five 

 pairs of perforated radial buccal plates close to the margin of the mouth, 

 and the rest of the membrane is either soft or irregularly plated. The 

 same radial perforated plates are present in Clypeastroidea with the addition 

 of a single interradial between each pair. The peristome becomes more or 

 less excentric during growth in Petalosticha and in the most typical family, 

 Spatangidae, transversely elongate as is the mouth itself. It has imperforate 

 plates. A membranous area or periproct covered by irregular plates sur- 

 a For the ' lag,' see Martin Duncan, A. N. H. (5), xvi. 1885. 



