212 ROBERT TRACY JACKSON ON ECHINI. 



Adult cidarids differ from the young principally in having low instead of high ambulacral 

 plates, the ventral border of the corona resorbed, many rows of ambulacral, also (with rare 

 exception) interradial non-ambulacral plates in the peristome. Many rows of ambulacral peri- 

 stomal plates is distinctly a Palaeozoic character (Plate 23, fig. 1), and some genera as Archae- 

 ocidaris have non-ambulacral plates as well. The oculars of adult cidarids may be all exsert as 

 in the young, or one or more to all insert, as shown here. The periproct is plated with numer- 

 ous heavy plates as in the Perischoechinoida. The lantern of the adult is nearly vertical 

 (Plate 2, figs. 7-10), instead of inclined, with a very shallow foramen, and short interpyramidal 

 muscles. The perignathic lantern muscles are all inserted on the elevated apophyses, a feature 

 peculiar to this order only (text-fig. 222, p. 193). The absence of pits in the top of the pyra- 

 mids is a feature like the Perischoechinoida but unlike the Centrechinoida. Teeth are grooved 

 as in the Echinocystoida, Perischoechinoida, and Aulodonta. Primary spines are strikingly 

 developed, naked of epidermis, with a cortical layer and a collar about the base, thus differing 

 from the next order. The presence of frondescent Stewart's organs and the absence of peri- 

 stomal gills and sphaeridia are distinctive characters of the order. The Cidaroida present 

 distinctly a combination of Palaeozoic with modern characters. 



Apparently only two species in the Palaeozoic are definitely referable to the Cidaridae, the 

 Lower Carboniferous Miocidaris cannoni sp. nov. from America (p. 247), and the Permian 

 Miocidaris keyserlingi (Geinitz) from Europe (p. 245). 



The Centrechinoida as an order differs from the Cidaroida in that the ambulacral plates 

 are usually compound; the peristome (excepting the Echinothuriidae) has only one row of 

 primordial ambulacral plates, with non-ambulacral plates. The lantern has a deep foramen 

 magnum, the tops of the pyramids are pitted, and auricles are developed, on which are inserted 

 the retractor muscles of the lantern. Peristomal gills and sphaeridia are developed. The 

 young of the Centrechinoida (Plate 3, figs. 9-11) make a close approach to cidarids in having 

 simple ambulacral plates, and large central spines and tubercles in the middle of the inter- 

 ambulacral plates. Further, the young have a single primordial interambulacral plate in each 

 area, peristome covered with ambulacral plates only, inclined lanterns with long interpyramidal 

 muscles recalling young cidarids, and the characters seen in the adults of certain Palaeozoic 

 types, as previously discussed. Of the three suborders of the Centrechinoida, the first has the 

 most characters like the Cidaroida, and the last has the fewest (p. 183). 



The suborder Aulodonta, while having the characters of its order, makes an approach to 

 the Cidaroida in that the teeth are grooved, epiphyses narrow (text-fig. 211, p. 184), and 

 primary tubercles perforate. Some members also approach the Cidaroida in possessing simple 

 ambulacral plates, or Stewart's organs, or many rows of ambulacral plates in the peristome. 



The Hemicidaridae being wholly a fossil group, some characters are wanting. The peri- 

 stome is unknown. They are primitive in that oculars are mostly all exsert, regressive in that 



