Studies on the EcJtinoidea Holectypoida. 213 



Morphological Studies on the Echinoidea Holectypoida 

 and their Allies. 



By Herbert L. Hawkins, D.Sc, F.Gr.S., Professor of Geology, 

 University College; Reading. 



XII. PSEUDOPYGASTER, A NEW TYPE OF THE 

 ECHINOIDEA EXOCYCLICA FROM THE MIDDLE LIAS 

 OF PERSIA.. 

 PLATE IX. 

 (i) Introduction. 

 T lASSIC Irregular Echinoids are rare. The interest attaching 

 -L^ to them is exceptionally great, and is enhanced by this rarity. 

 Separation of Exocyclic from Endocyclic types can hardly have 

 preceded the Rhaetic period ; hence each Irregidar form fouLd in 

 the Lias might be expected to display evidence as to the origin of 

 its sub-class. It cannot be claimed that the few such genera known 

 fulfil expectation. Plesiechinus (represented by P. reynesi) occurs 

 in the " Charmouthian " ; it is but little removed from " Regularity ", 

 but the Bajocian species P. ornatus (which is locally abundant) 

 seems " just as good " in that respect. Holectypus appears in the 

 Toarcian with H. conquensis ; its occurrence at this horizon gives 

 a striking illustration of morphogenetic acceleration, and thereby 

 a negation to hopes of " genealogical " evidence. Species of 

 Galenrpygus arc recorded from the Upper Lias ; orthodoxy would 

 probably repeat the preceding sentence here, but vide infra. The 

 case of Collyrites, recorded from deposits of Liassic age, must surely 

 be compared with that of Holectypus ; for every feature of the 

 Collyritidae is in violent contrast with " Regular " structures. 



Loriolella should have accompanied these four genera in the 

 foregoing paragraph, according to its founder, Fucini ; but for 

 reasons set forth below it deserves separate consideration, Not 

 the least of these reasons is the discovery (by Lambert) that the 

 peripioct was enclosed by the apical system, so that Loriolella is 

 not an Irregular Echinoid. Lambert places it near Mesodiadema and 

 Orthopsis. Fucini's figures of L. ludovici are somewhat obscure ; 

 while comparison of his diagnosis of the genus with that given by 

 Lambert & Thiery (1911, " Essai de Nomenclature," fasc. iii, p. 195) 

 arouses a passing suspicion that wholly different types are con- 

 sidered by tlie two writers. However, after careful study of 

 Fucini's account, I feel convinced that his specimen was described 

 7ipside-down. Only on such an assumption is it possible to reconcile 

 the two generic diagnoses : without it Loriolella would be more 

 extraordinary than its author realized. Then the " slightly con- 

 cave upper surface " becomes the adoral surface ; the ambulacra, 

 " widest at the apex and gradually tapering towards the peristome," 

 are inverted to conform with those of all other Echinoids ; the 

 interradial tubercles, " fairly large on the upper surface, becoming 



