84 



ROBERT TRACY JACKSON ON ECHINI. 



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

 Pkurechinus 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 type. In the Centrechinoida the non-ambulacral buccal 

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

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

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



n 



TEXT-FIG. 57. Toxopneustes variegalus (Lamarck). Tampa Bay, Florida, tliam. 70 mm. R. T. J. Coll., 784. X 4. 

 Peristome with ten ambulacral and many non-ambulacral plates. 



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. 



In Dermatodiadema (text-fig. 51), as shown by Mr. Agassiz (1883, p. 26, Plate 9, fig. 5; 

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

 considerable part of the buccal membrane, a retention of a primitive character. 



