GElfEBA AND GUIOFPS OF THE ECHIJSTOIDEA. 21 



therefore four vertical rows of pores two on eacli side of a median 

 groove. 



Interradia with numerous, 6-8, vertical rows of scale-like 

 lozenge-shaped or irregular plates, each with a small primary 

 tubercle surrounded by a scrobicule ; granules present, and the 

 spines short, sharp, striated, and serrate. The areas appear to 

 unite apically. 



Peristome central, actinal, small, pentagonal or stellate, sur- 

 rounded by marginal ambulacral and interradial plates; dense 

 sheaves of short spines at the edge. Jaws highly developed, five 

 pyramids ; the inner, oral surface of the pyramids with strong 

 spines* or with striated chisel-shaped teeth. Periproct large, 

 on a low pyramidal protuberance in an interradium, at about 

 one third of the diameter of the test from the peristome. 



Fossil. Upper Silurian : Scotland. 



II. 



Remarks upon the Subclass Euechinoidea, the five Orders, definitions, 

 and of the Suborders of the second Order. Order I., the Cidaroida ; the 

 Family Cidaridas. Section I. Genus Oidaris (note on the classification). The 

 seven divisions of the genus ; definitions ; Subgenus Goniocidaris. Other genera. 

 Section II. Order II., the Diadematoida. The Suborder Streptosomata ; con- 

 siderations regarding the anatomy and classification. The Family Echinothu- 

 ridse, and Subfamilies Pelanechininse and Echinothurinse ; the description of the 

 genera. 



II. Subclass EUECRINOIDJEA. 



The classification of the Euechinoidea, like that of many of the 

 great groups of Invertebrata, can be generally natural and 

 definite ; but an artificial method seems to be necessary, occa- 

 sionally. Some of the " Orders " are well defined by characters 

 of considerable anatomical and physiological value, but others 

 are of unequal relative value, are perhaps too comprehensive, and 

 Suborders have to be founded. 



The Orders Cidaroida and Clypeastroida, amongst the Grnatho- 

 stomes, are well defined, but the discovery of the Drs. Sarasin 



* The spines at the oral surface of the pyramids are accidental, and the 

 result of crushing. 



