84 



ROBERT TRACY JACKSON ON ECHINI. 



as known, Dr. Mortensen (1904, p. 78) gives a remarkable exception, for he says that in 

 Pleurechimis doderleini Mortensen there are only five buccal ambulacral plates, a "unique 

 feature." This may fairly be considered an exceptional departure from the ten-plate system 

 rather than instituting a distinct tj'pe. In the Centrechinoida the non-ambulacral buccal 

 plates are usually small, thin, and scale-like, often widely scattered, and maj' be mere granules, 

 or they may be quite absent, as in Echinus magellanicus and some of the Temnopleuridae. 

 They may, however, be relatively thick and solid and form a pavement-like plating of the whole 



TEXT-rro. 57. — Toxnpncusles vdrier/aliis (hrinvMck) . Tampa Bay, Florida, diam. 70 mm. R. T. ,1. Cull., 7S-J. X 4. 

 rpristome with ten ambulacral and many non-ambulacral jjlatcs. 



area, as in Toxopneustes variegatus (text-fig. 57). Here the primordial ambulacral plates fall 

 in sets of twos in the radial areas with a rather wide interspace between the sets. The pores 

 of these plates are superposed dorso-ventrally as usual in peristomal plates. 



Ill Dermatodiadema (text-fig. 51), as shown by Mr. Agassiz (1883, p. 20, Plate 9, fig. 5; 

 1904, p. 61, Plate 25), the iiiimordial ambulacral plates are extraordinarily large and fill a 

 considerable i)art of the buccal membrane, a retention of a pi-imitivc character. 



