422 PKOP. p. M. DUXCAN OH THE STETTCTITEE OE 



more plates, all more or less departing from their original shape. 

 Growth near the unyielding peristome produces shifting of the pori- 

 ferous parts of plates, and hence the apparent confusion of the pairs 

 of pores in that region in so many genera. It must be understood 

 that the confusion is only apparent, for Loven has explained it, and 

 no additional pairs of pores intercalated there during growth. 



The physiological importance of the ambulacra to the Echinoid 

 cannot be over-estimated ; for the peripodia support the prehensile 

 or motor tentacles, and in the ArbaciadsB and the Diadematidae the 

 tentacles which are above the ambitus have a non-prehensile and 

 branchial function. The number of tentacles placed within a given 

 area is therefore of classificatory as well as of physiological im- 

 portance, and this number bears a definite relation to the number, 

 kind, and shape of the plates which constitute the ambulacra. 



The Echinoidea with the simplest ambulacra are found in the 

 lower Secondary deposits, and it is advisable in the present inquiry 

 to commence with the description of some of the earlier forms of 

 the Diadematidae. It is proposed to consider the genera Remijjedina, 

 Wright, Pseudodiadema, Desor, Pedina, Agass., StomecJiinus, Desor, 

 Eemicidaris, Agass., Biplopodia, McCoy, and Cy^Jiosoma, Agass. 



III. DESCRIPTIOlfS OE THE AmBTILACEAL PlATES OE THE GeIS'EEA 



Hemipedina, Psetjdodiadema, Peddsta, STOMECHiDfus, Hemicidaeis, 



DiPLOPODIA, AND CXPHOSOMA. ThE jSTeCESSITY EOE A NEW GeNIJS, 



Plesiodiadema. 



Genus Hemipedina, Wright, 1855. 



The diagnosis of this genus will be found in the ' Monograph of 

 the British Eossil Echinodermata,' Pal. Soc. Lond. 1855, p. 143. 



Dr. Wright made the following remarks concerning the affinities 

 of this genus with other genera : — " Hemijpedina is related to Pseudo- 

 diadema in having the pores unigeminal and the tubercles per- 

 forated ; but it is distinguished from Pseudodiadema by the absence 

 of crenulations from the summit of the bosses. It is related to 

 Pedina in possessing perforate and uncrenulate tubercles ; but it is 

 distinguished from that genus in having the pores unigeminal, 

 Pedina having the pores trigeminal as in EcJiinus. The elements of 

 the disk are likewise more largely developed. Eemipedina is related 

 to EcJiinopsis,'but is distinguished by the narrowness of the ambulacral 

 areas, the general depressed form of the test, the shape of the mouth- 

 opening, and the deep decagonal lobes of the peristome (that of 

 Ecliinopsis being almost deprived of incisions), together with the 

 greater size and development of the elements of the apical disk." 

 In the drawings of a species of Eemipedina^ Dr. Wright noticed 

 the long slender needle-shaped spines with fine longitudinal stria- 

 tions. He states, moreover, that the optic pore is in the centre 

 of the radial plate. 



Eemipedina Jardinii^ Wright, has a considerable series of low and 



