THE AMBULACRA OF FOSSIL ECHIXOIDEA. 451 



compound plate, the middle plate beiug the largest, and the two 

 others are smaller and become demi-plates in consequence of the 

 growth -pressure exercised b}^ the great tubercle of the compound 

 plate. (JoLirn. Linn. Soc. Zool. vol. xix. p. 25, 1885.) 



4. The Echinoid type. This has primaries near the radial plate 

 and then compound plates are seen of three or more plates combined. 



The middle plates are demi-plates and the primaries are aboral 

 and adoral, or all the aboral xDlates may be demi-plates. (Loven, 

 Etudes, and ITonogr. of the Possil Echinoidea of Sind, Ease. Gaj 

 Series, Pal. Indica, Ser. xiv. 1885, Duncan & Sladen, Hipponoe.) 



5. The Cyphosomoid type. This unites the Echinoid, the Diade- 

 matoid, and the next or diplopodous type. 



6. The Diplopodous type. The primaries near the radial plates 

 are, in young forms as well as in the adults, arranged in a double 

 row ; and this condition reaches to a greater or less distance towards 

 the ambitus or even to the peristome. There is great diminution orf 

 the height or absorption of the non-poriferous parts of some plates. 



It is evident that while the Cidaroid type never varies, the 

 Diadematoid and Arbacioid types tend to the Echinoid type in some 

 instances on account of the formation of one or more demi-plates in 

 a compound plate. 



It is also interesting to notice that the Arbacioids, which came 

 later in time than the Diadematidse, have the usual simple Cidaroid 

 arrangement in the young plates near the radial plate, and at some 

 distance down a plate or two on the Diadematoid type. Then come 

 the true Arbacioid compound plates. 



The demi-plate came in with the Pseudodiademata, and became of 

 importance in the construction of the compound plates of Ocelo- 

 jpleurus and the later genera of Arbacioids. 



Finally the differences in the construction of the ambulacra 

 necessitate the separation of the genera Plesiodiadema and Diph- 

 podia from Pseudodiadema. 



The only notice that I have been able to discover of the re- 

 markable disposition of the plates of the Diadematidse is in the 

 description of Heterodiadema ouremense by De Loriol (Eecueil Zool. 

 Suisse, t. i. no. 4, Sept. 1884, p. 626). There is a drawing given 

 and a description of the triple plate, and they conform to the type 

 of the true Diademata. I did not see this communication of M. de 

 Loriol until the essay I have read before the Geological Society was 

 completed. Cotteau gives indications of some sutures ia many of 

 his plates on the Echinoidea, but he does not describe the sutures or 

 pay attention to them. 



DESCRIPTION OF THE FiaUEES IN THE TEXT. 



Fig. 1. Two compound plates of Strongylocentrotus, after Loven (p. 421). 



2. A compound plate of Hemi^edina Jardini, Wright (p. 423). 



3. A compound plate of Hemipedina marchamensis, Wright (p. 424). 



4. A compound plate of Hemi'pedina tuhercidosa, Wright (p. 425). 



5. Part of the ambulacrum near the ambitus of PseudodiadeTna 



s^phcericum, Lamk. (p. 428). 



