55 



cidaridœ certainly have to be referred to this group also, since they likewise have 

 unkeeled teeth, diadematoid ambulacra and perforate tubercles. 



The Saleniidœ, Arbaciiilœ and Phymosomatidœ agree in having imperforate 

 tubercles and keeled teeth. The Saleniidœ have simple ambulacra; in the two other 

 families the structure is diadematoid. These three families are evidently related, 

 especially the Arbaciidœ and Phymosomatidœ. The Saleniidœ on account of their 

 other peculiarities (apical system, spines of cidaroid structure) form a special group, 

 but it is perhaps not unreasonable to see their nearest allies in the Arbaciidœ. If 

 the fam. Stomechinidœ Pomel be rightly maintained, it will probably have its place 

 near the Arbaciidœ. 



The rest of the families of regular Echinids all agree in having echinoid 

 ambulacra, imperforate tubercles and keeled teeth. These families then evidently 

 form one group, as has, indeed, been commonly accepted. 



The result of the grouping of the families after the structure of ambulacra, 

 tubercles and teeth is thus seen in no way to violate evident natural relations. All 

 the diadematoid genera remain in one group and all echinoid genera likewise 

 remain together. It can thus scarcely be said to be an unnatural classification. 

 But then the uniting of Arbaciidœ and Phymosomatidœ (and Stomechinidœ) must in 

 all probability also be the natural grouping of these families, which have been so 

 differently placed in the previous systems; likewise the referring of the Hemicidaridœ 

 to the group of the Diadematids must certainly be correct. My system of the 

 regular Echinids then looks as follows, the Palæecliinids being partly left out of 

 consideration : 



Order 1. Entobranchiata. 



Suborder. Cidaroida. Both ambulacral and interambulacral plates continuing 



over the peristome. 

 Fam. 1. Archæocidaridæ. 



— 2. Cidaridæ. 

 Order 2. Ectobranchiata. 



Suborder 1. Streptosomata. Ambulacral plates covering the peristome; no 



large „buccal" plates, the first pair of tubefeet 

 not developed as larger „buccal" tubefeet. 

 Fam. 1. Lepidocentridæ. 



— 2. Echinothuridæ. 



Suboi-der 2. Stereosomata. Only 5 pairs of ambulacral plates, the „buccal" 



plates, on the peristome, provided with tubefeet 

 mostly larger than those of the test. 

 Tribus 1. Diademina. Ambulacra diadematoid; tubercles perforate; teeth 



unkeeled. 



