Studies on the Echinoidea Holectyjioida. 219 



The contrast between Pseudopyg aster and the Cidaridae is great, 

 but there are three counts on which comparison is profitable. 

 The unituberculate nature of the interainbulacral plates is one 

 of these resemblances, but this is somewhat discounted by the 

 exceedingly non-Cidarid proportions of the tubercles. However, it 

 suggests a lack of progress in this feature of Pseudopygaster. A 

 more striking comparison is found in the narrowness of the ambulacra 

 and the relatively great number of ambulacral plates. Ten 

 ambulacrals to each interambidacral (the condition at the ambitus) 

 represents a proportion about twice that normal to '" Diadematoid " 

 •or " Exocyi'Iic " types (excluding forms with elaborate plate- 

 combination), while the extreme narrowness of the areas enhances 

 their resemblance to Cidarid ambulacra. Again, the large size of 

 the apical system might indicate Cidarid affinities. 



Lastly, the assumed magnitude of the apical system is certainly 

 suggestive of " Calycine " relations, but in the absence of evidence 

 as to the composition of the region nothing further can be said 

 in this connexion. 



Plesiechinus, in respect of its small departure from " Regularity " 

 and its chronological ])osition, would seem to be the most 

 appropriate of known Exocycloida for comparison. Further, it is 

 regarded almost universally as the probable ancestor of all 

 Irregular Echinoids. But size and periproct position are the only 

 features where comparison with Pseudopygaster is possible, and 

 even the latter of these shows m.any important differences. Con- 

 trast is the doininant impression when the two genera are considered 

 together. Plesiechinus has a large, gaathostomatous peristome 

 deeply embayed by branchial incisions ; in Pseudopggaster the 

 corresponding qualities are small, almost certainly edentulous, and 

 practically negligible notches for branchiae. The ambulacra of 

 PlesiecJiimis are almost one-third the width of the interambulacra, 

 and consist of plates (mainly primaries) with well-defined perforate 

 tubercles set in ternary arrangement ; Pseud opygaster shows 

 ambulacra hardly one-tenth of the ambital width of the intervening 

 areas, built of primaries (perhaps not entirely so near the peristome) 

 with inconspicuous granules fairly evenly distributed, and no 

 tubercles. Plesiechinus has several coluinns of tubercles in the 

 interambulacra in addition to a practically median " Cidaroid " 

 column ; Pseudopygaster has but the single column of interradial 

 tubercles, eccentric in position. To summarize, Plesiechinus is 

 a typical Holectypoid, and Psexidopygaster is just as definitely not 

 a member of that order. 



Excluding an imperfectly known member of the Colly ritidae, 

 Galeropygus remains the sole Liassic genus of Irregular Echinoids 

 with which comparison is feasible. And it must be admitted that, 

 at first sight, there appears little promise of such a comparison 

 proving fruitful. However, the hemispherical outline of 

 Pseudopygaster proves, on closer analysis, the only striking contrast 



