in some Fossil Echinoidea. 259 



Pygaster semisuxcatus (Phill.). PI. XIII, Pig. 3. Inferior Oolite. 

 Although, not a Cassidulid, and possessing no true phyllode, 

 Pygaster, the earliest of the Irregular Echinoids, shows the primitive 

 condition from which this structure was evolved. The ambulacrum 

 tapers gentry towards the mouth, but occupies a considerable part of 

 the margin of the peristome. The ambulacrals remain simple priinaiies 

 from the apex to a point on the adoral surface about half-way between 

 the ambitus and the mouth. There are sometimes slight traces of 

 a sub-petaloid differentiation of the pores on the adapical surface. 

 When the plate-crushing commences it follows the usual Holectypoid 

 plan, every third plate gradually becoming a demiplate. This is 

 plate o (see PI. XIII, Fig. 2). In this way plate a, below the demi- 

 plate, makes up a larger part of the perradial suture than before, and 

 appears proportionately narrower towards the interradial margin. 

 Plate b remains practically unaffected, and the resulting appearance 

 of the ambulacrum is somewhat like that of the greater part of the 

 ray. of Conulus. There is a tendency for the pore-pairs to become 

 rather widely spaced in the peristomial parts of the ambulacrum in 

 Pygaster, and they rarely show any departure from a uniserial 

 arrangement. The traces of displacement which they may exhibit 

 result from a slight perradiad shifting of the pores of plate a, those of 

 demiplate c becoming more approximated to the interradial margin of 

 the ambulacrum. 



Galeropygus agariciformis (Forbes). PI. XIII, Fig. 4. 

 Inferior Oolite. 



Galeropygus marks a considerable advance on Pygaster in the 

 progress towards irregularity. The ambulacra, which remain apetaloid 

 throughout, are very much narrower in proportion, and, owing to the 

 small size of the peristome, taper considerably in their adoral parts. 

 Perhaps on account of this lateral compression the vertical plate- 

 crushing is carried to a further extent than its contemporary. The 

 first reduction of plate c takes place in a position similar to that in 

 Pygaster, but almost before this plate becomes a demiplate the 

 interradiad extension of plate a becomes pinched to a narrow strip. 

 In some cases it is entirely cut off from the interradial margin, and 

 plates b and c meet round it. In the more extreme cases plates a and 

 c are almost on the same horizon, and together form a rough parallelo- 

 gram, with a diagonal sloping interradially and adorally. This crushing, 

 which is the true phyllode structure in a simple form, becomes 

 reduced in intensity near the margin of the peristome, where the plates 

 usually become somewhat irregular primaries. The pore-pairs take 

 on a decidedly triserial arrangement. The pores of plate a are, by the 

 more or less complete atrophy of the interradiad portion of the plate, 

 compelled to take up a position which is often nearer to the perradial 

 than to the interradial suture. The pores of plate b, as if in sympathy, 

 move inwards to about half this extent, while those of demiplate c are 

 perforce retained in their usual marginal position. A triad is thus 

 composed of the pore-pairs of plates a, b, and c in descending order. 

 Galeropygus agariciformis has all the essential features of phyllode 

 structure, although the ambulacra are neither expanded nor depressed. 



