GENERA AND GROUPS OF THE ECHTNOIBEA. 23 



the genus in a separate Suborder is not advisable ; and. it is 

 perhaps best to enlarge the defiaitioa of the Order, wMcli con- 

 tains Solectypus as a type, so as also to include the genera with 

 perignathic girdles of more or less continuous collar-shape, without 

 jaws. At present our knowledge of the genera Galeropygus 

 and FacJiyclypeus is so defective that they must remain incertoe 

 sedis. 



The abolition of the toothless Irregulares (Esocyclica) as a 

 group assists the definition of the Cassiduloidea, for Phyllo- 

 clypeus, Conolampas, and even JEckinoconus can enter it, only 

 the latter genus being, possibly, somewhat artificially placed. 

 The separation, made many years since by Desor, between 

 the Cassiduloids with a floscelle and without that important 

 peristomial character, holds good, and the only difl&culty in the 

 taxonomy is to place the few geuera which have, besides some 

 Cassiduloid characters, the dissimilarity of the ambulacra charac- 

 teristic of the Spatangoida, and variable apical systems, such as 

 Eolampas, Archiacia, Glaviaster, Asterostoma, &c. These genera 

 are aberrant, yet they link the Cassiduloidea with the Spatau- 

 goidea in an Order. They form a new Family, the Plesio- 

 spatangidse. 



The classification of the great group of Spatangoidea — a Sub- 

 order — has been rendered difficult in consequence of the discovery 

 of the abyssal forms. Under the handsof Loven*and A. Agassizf 

 the suborder has become manageable, and its Families are those 

 of the Ananchytidse, Spatangid®, Leskiidae, and Pourtalesiidae. 

 The Spatangidae contain four well-marked types, but they do not 

 appear to be worthy of subfamily distinction, for their separating 

 characters are rather artificial and are of unequal value. The 

 divisions are those insisted upon by Loven, namely, the genera j 

 without fascioles, those without a subanal fasciole, those with \ 

 a subanal fasciole, and those which are apetaloid. These are 

 the divisions of Adetes, Prymnadetes, Prymnodesmia, and 

 Apetala. 



The attempt is made, in this revision, not to overvalue the 

 characters of perforation and crenulatiou or of the plain and 

 imperforate nature of primary tubercles. The physiological value 

 of crenulation is very slight, it often occurs or is absent in the 



* ' fitudes ' and ' Pourtalesia.' 



t Eeport on the ' Challenger ' Echini. 



