492 MR. HERBERT L. HAWKINS ON 



Desor (21) tinder the name of " JEchmoconus." " Gcderites " 

 roemeri d'Orb., on which the genus was founded, seems to be 

 congeneric with the " Echinoconus abbreviatus " and " orhignyanus" 

 of the Upper Chalk of Norfolk. The differences betw^een these 

 species and a typical Conuhos are manifold, and, as the name 

 Echinoconus cannot be retained for them, I have distingviished 

 them as Gonulopsis. The tuberculr.tion of Conulopsis is irregular, 

 and the tubercles of the adapicai surface have deeply sunken 

 scrobicules. The ambulacra are composed of primaries through- 

 out, the adapicai pore-pairs being almost subpetaloid ; while 

 round the peristome the interambulacra are raised into definite 

 " bourrelets." The general facies of Conulopsis is similar to 

 that of Caratomus (the latest discussion of this genus being by 

 Schlueter, 54). It is possible, however, that some real genetic 

 relation may exist between the later Conuli and Conulopsis, and 

 that the resemblance of the latter genus to Caratomus may be 

 deceptive. Even if Conulopsis is a descendant of Conulus, it 

 is certainly not an Holectypoid. It Avould show a development 

 which wovild have a peculiar interest when coanpared with the 

 development of Conoclypeus from. Discoidea. The same loss of 

 regularity in the tuberculation is seen, and the ambulacral plates 

 have become restored to their primary state. (The large polygonal 

 ambulacrals of the adoral surface of Comdopsis are strikingly 

 similar to those of a Clypeaster or of a Spatangid.) The develop- 

 ment of a subpetaloid character in the adapicai parts of the 

 ambulacra would be comparable in the two genera, while a 

 similar correspondence is shown in the peristomial " bourrelets." 

 Only the position of the periproct (almost marginal in Comdopsis), 

 and the presence of strong jaws in Conoclypeus, would tend 

 to sepai'ate the two genera. These last features would be 

 definitely due to the characters of the ancestor, Conulopsis 

 agreeing in them with Conulus, and Conoclypeus with Discoidea. 



Comdopsis and Conoclypeus would then mark parallel accelera- 

 tions from diflferent branches of the Holectypoid stock. At 

 present, however, I do not feel satisfied that the genetic connection 

 between Comdus and Conulopsis exists, but I have connected the 

 two by a broken line in the table. 



6. Discoidea and the Clypeastroida. 



The similarities of structure that link the Discoidiinte with 

 the Clypeastroida are many and of fundamental importance. A 

 circular outline ; an invaginated peristome ; an infra-marginal 

 periproct ; a madreporite scattered over five genitals, all of which 

 may be perforated by a genital pore ; and internal buttresses to 

 the test : are common to most genera of the Clypeastroids, and 

 are diagnostic features in Discoidea. The jaws in this genus 

 are strong, in view of its late appearance among the Holectypoida, 

 and, although conforming more to the " Regular" than to the 

 Clypeastroid type, may well have assumed a more expanded 



