NO. 3 CASSIDULOID ECHINOIDS — KIER 7 



this current. Furthermore, a double-pored ambulacrum is structurally 

 weaker than one with a series of single pores. 



Almost simultaneous with the reduction from two pores to one, 

 another very significant and important feature appears: the first 

 buccal pores, through which extended tube-feet that were presumably 

 sensory in function and aided in the detection of food. In general, 

 species with double pores lack buccal pores, and species with single 

 pores have them. There are few exceptions to this combination, and 

 most of these exceptions occur in species that lived during the transi- 

 tional period when the first single pores appeared, such as Hypopyg- 

 iiriis gaudryi Gauthier (chart 2, fig, P), in which there are buccal 

 pores but the ambulacra are all double pored ; Gcntilia tofileltensis ? 

 Lambert (chart 2, fig. O), with single pores but no buccal pores, and 

 Gent ilia syriensis Kier (chart 2, fig. O), with minute buccal pores, 



PHYLLODES 



The phyllodes in the cassiduloids can be divided into two types: 

 the nucleolitid and the pygurid. The pygurid type is found in many 

 species of the Galeropygidae and the Clypeidae, and is particularly 

 well exhibited in Pygnrus. In this genus, there is a remarkably 

 consistent trend in the evolution of the phyllodes, with a broadening 

 of the phyllodes, a reduction in the number of pore pairs, and an in- 

 crease in the distance between the pores and the edge of the peristome. 

 This trend is illustrated in chart 3. In order to assure objectivity in 

 this study, I have included on this chart a drawing of a phyllode of all 

 the well-dated species of Pygnrus in which this area has been figured. 

 In the Middle Jurassic species of Pygnrus, such as P. (Mepygurns) 

 deprcssns Agassiz (chart 3, fig. a) from the Bathonian, the phyllodes 

 are very long, not broadened, and have many pore pairs arranged in 

 three series in each half -ambulacrum. The pore pairs extend almost to 

 the edge of the peristome, Callovian species such as P. (Mepygnrus) 

 marmonti (Beaudouin) (chart 3, fig. b) and P. (Mepygnrus) sp. 

 (chart 3, fig. c) from Madagascar have similar phyllodes except for 

 a slight broadening of the phyllode near the peristome. By Lusitanian 

 time, the phyllodes have changed considerably. In P. (Pygnrus) 

 gcryvilliensis Peron and Gauthier (chart 3, fig, e) and P. (Pygnrus) 

 hlumenhachi Kock and Dunker (chart 3, fig. d) the phyllodes are 

 greatly broadened, with the pore pairs shifted more laterally to the 

 edge of the ambulacrum with a wide area between the two inner series 

 of each half -ambulacrum. The pore pairs terminate far from the 

 edge of the peristome, and there are fewer pore pairs in the inner 



