THE AMBULACRA OF FOSSIL ECHINOIDEA. 427 



the regular Echinoidea, the forms of which have three pairs of pores 

 only to each ambulacral plate. 



Test of moderate or small size. The tubercles are of the same 

 size in both areas, and are crenulate and perforate. The tubercles 

 either only form two rows in the interradia, and may be without 

 secondaries, or they may be arranged in four or even six rows. 



Poriferous zones simple. Spines smooth or faintly striated. 



Eange from the Lias to the Cretaceous inclusive. 



Desor notices that the forms thus diagnosed were termed, in the 

 ' Catalogue Raisonne,' " Diadema,"' and were placed alongside of the 

 recent species which bear that generic appellation. But besides 

 being smaller than the modern forms, there is the character of the 

 latter which relates to the spines to be considered, according to Desor. 

 He reminds us that the spines in the modern species of the genus 

 Diadema are verticillate in their striated ornamentation. 



Under the belief that this distinction was of great classificatory im- 

 portance, he separated the species which are found fossil, as belonging 

 to the genus Pseudodiadema, and took two species as typical of two 

 divisions of the genus — Pseudodiadema hemisphoericmn (Cidarites 

 pseudodiadema of Lamarck, or Diadema pseudodiadema, Agass. & 

 Desor) and Pseudodiadema mamillanum, Eomer. The first he con- 

 sidered represented the group with several rows of secondary tubercles 

 in the interradia ; and with the latter he associated all the forms 

 with only two rows of tubercles in an area. 



It will be noticed that in the diagnosis Desor did not find a place 

 for certain species which have been admitted since, and which have 

 a doubling of the pairs of pores in the region above the ambitus. 

 McCoy had separated the species with doubling of the pairs near 

 the apex from Diadema, and had founded the genus Diplopodia for 

 them. Wright, however (op. cit. p. 109), adds to the generic cha- 

 racters of Pseudodiadema, " the pores in one section are unigeminal 

 throughout, and in another section they are bigeminal in the upper 

 part of the zones." 



The same autbor states that Pseudodiadema differs from Diadema 

 in having solid spines, with a smooth surface, the sculpture, in most 

 cases, consisting of microscopic longitudinal lines. He also remarks 

 that the genus diff'ers from Cyphosoma in having the tubercles 

 always perforated. The necessity for allying Cyphosoma and thus 

 adding to tbe confusion is a consequence of admitting forms with 

 doubling of the pores into the genus Pseudodiadema. Wright 

 noticed the genus Diplopodia, and remarks as follows in placing it 

 on one side : — ^^ C ceteris parihus, the crowding together of a greater 

 number of pores in a zone is, at most, a sectional, and can never form 

 a stable generic character, inasmuch as it is subject to great varia- 

 tion in the diplopodous species themselves, and is, moreover, often 

 only an adult development." 



Having studied the morphology of the ambulacra of the recent 

 Diadematidse (Journ. Linnean Soc, Zool. vol. xix. p. 95) I was 

 greatly impressed with the results of a careful examination of many 

 forms of the allied genus Pseudodiadem>a. I came to the conclusion 



