GEITEEA A"ND GROUPS 01" THE ECHINOIDEA. 57 



rated and crenulated tubercles, and witb. expanded bases, largest 

 at and above the ambitus. 



Peristome smaller than the apical system, with small branchial 

 incisions and bifid branchiae ; ten very large spiued buccal plates. 

 Teeth grooved. Spheridia large and extending high up on the 

 ambulacra. Spines hollow, verticillate, straight and curved, some 

 large and very long, others similar and small. Pedicellariae various, 

 some with large glands upon the stem. 



Becent. Caribbean Sea, 95-1200 fms. ; IN", part of the South 

 Atlantic, and also the Chili coast, 356-2225 fms. ; Philippine Sea, 

 100-1700 fms. 



The ^Family Diadematidse was well formulated by Cotteau and 

 Wright, and the arrangement of the genera, by the existence 

 or not of crenulation and perforation of the primary tubercles, 

 has been very popular and useful ; but ib is too artificial, for 

 the physiological importance of the superficial structures of the 

 tubercles is exceedingly small. Careful consideration proves 

 that many genera have been placed in the Family which 

 can hardly remain, and they have now found places in the 

 Eamilies of Heinicidaridse, Temnopleuridse, Arbaciidse, and 

 Cyphosomatidse. 



Many genera have been introduced since Cotteau founded the 

 Pamily, and it is now fairly homogeneous, if they are arranged 

 with their fellows on the principle of the structures of the ambu- 

 lacra being considered of primary taxonomic importance. It is 

 the group of genera which is characterized by having numerous 

 small tubercles placed actinally and at the ambitus, but then 

 ceasing more or less, which has given much trouble. Such genera 

 are Codiopsis, Gymnocliadeina, Plistophyma, and PolycypJms. 

 Again, Orthopsis and its allies are difficult to classify upon the old 

 lines of the value of crenulation and perforation. It may be 

 stated, as a general truth, that if these genera be classified by the 

 ornamentation of the tubercles, groups possessiug very diverse 

 ambulacra will be associated. 



The Family separates by the new method into subfamilies. 



A most unfortunate decision of Desor's has made palaeonto- 

 logists and students of the recent fauna look upon some of the 

 most important species of one genus in a very diflFerent light. 

 Desor introduced the genus Pseudodiadema to take the place of 

 fossil Diademce. The only distinctions between the recent genua 



