THE PERISTOME. 85 



A fifth type of perif^toinc is seen in clypeastroids (Ecliinaiiu'linius, text-figs. 52, 53), in 

 which the primordial ambulacra! plates no longer exist as part of the buccal system, but have 

 become transferred to the basicoronal row of the corona, where they are typically retained 

 throughout life, alternating with the primordial interambulacral plates. The buccal membrane 

 proper in Echinarachnius and, as far as I have been able to ascertain, in other genera of this 

 suborder, is leathery without plates. 



Still a sixth type of peristome is seen in spatangoids (Echinocardium, text-fig. 54). In 

 these the primordial ambulacral i)lates are in the basicoronal row of the corona, as in clypeas- 

 troids. The buccal membrane, however, is covered with numerous thin, scaly plates which, 

 as in the Centrechinoida, cannot apparently be considered as ambulacral or interambulacral, 

 but as plates which developed in place and independently of the corona. 



In brief the six types of peristomes are: — 



1. Two rows of continuous ambulacral plates, — the primordial row and one in addition 

 (Bothriocidaris, text-fig. 40). 



2. Many rows of continuous ambulacral plates (Hyattechinus, Plate 23, fig. 1, Phor- 

 mosoma, text-fig. 43). 



3. Many rows of ambulacral with intercalated interradial non-ambulacral plates (Euci- 

 daris, text-fig. 46; Archaeocidaris, text-fig. 47; Melonechinus, text-fig. 48). 



4. One row of continuous primordial ambulacral plates around the mouth, and usually, 

 in addition, solid, scaly, isolated, or granular non-ambulacral plates ^ (Dermatodiadema, text- 

 fig. 51; Centrechinus, Salenia, Strongylocentrotus, text-fig. 50; Toxopneustes, text-fig. 57). 



5. No primordial ambulacral plates, as they are in the corona; peristome leathery with 

 no ambulacral or other plates (Echinarachnius, text-fig. 52). 



6. No primordial ambulacral plates, as they are in the corona; area of peristome with 

 scaly plates which are not referable to ambulacra or interambulacra (Echinocardium, text-fig. 

 54). 



The origin of the plates of the peristome is a matter of much interest. Without question 

 the primordial ambulacral plates, when they exist on the peristome (as they apparently do 

 in all Echini excepting the Exocycloida, where they appear typically as basicoronal plates), 

 originate in that area and entirely independently of the corona. In Bothriocidaris it is possible 

 that the second row of ambulacral plates, like the primordial ambulacral plates, originated on 

 the peristome and independently of the corona, but more ijrobably they were derived by 

 flowing down from the corona. 



When ambulacral plates other than primordial ambulacral plates exist on the peristome, 



' In some of the Temnopleuridae (A. Agassiz, 1872, p. 286; Mortensen, 1904, p. 80) and Eohinitlac (Doderlein, 1906, 

 p. 226) the non-ambulacral plates are absent. 



