THE ECHINOIDEA 305 



As is the rule among all thin-plated, flexible Echinoids, there is a marked 

 tendency to irregularity, especially shown in Pholidocidaris. Genera 

 Pholidoddaris and Lepidcsthes, both of Meek & Worthen, from the Lower 

 Carboniferous. 



ORDER 5. Plesiocidaroida, Duncan. 



Echinoidea Regularia Endobranchiata with a small rigid test , 

 peristome and periproct central and opposite. Periproct in the centre 

 of an apical system of large plates, which forms half of the whole 

 test. The ambulacral areas are short and biserial. Their plates are 

 all simple primaries. The interambulacra have each a single peristomial 

 plate. 



FAMILY 1. TIARECHINIDAE. Plesiocidaroida with desmactinic ambu- 

 lacra (. ambulacra continuous from peristome to apical system). Each 

 interambulacrum consists of four plates, viz. a single peristomial plate 

 and three tall, vertical plates in a horizontal row. Genus Tiarechinus, 



Tiarechinus princeps, Neuuiayr ; Trias, Tyrol. 1, from the side ; 2, from below ; 

 3, the apical system (magnified). 



Neumayr ; Trias, Tyrol. The figures (Fig. XVIII.) show its enormous 

 apical disc, small ambulacra, and vertical interambulacral plates. 



FAMILY 2. LYSECHINIDAE. Plesiocidaroida with ambulacra limited to 

 grooves on lower surface of the test Each interambulacrum begins with 

 a single peristomial plate succeeded by a row of two plates, and this by 

 one or more containing three plates. Genus Lysechinus, Greg. (34) ; 

 Trias, Tyrol. 



This small order includes the two most aberrant of all known 

 Echinoids. In Tiarechinus there are three vertical plates in each inter- 

 ambulacrum, while the calyx is much larger and more crinoidal in 

 aspect than in any other Echinoid. It has hence been regarded as an 

 argument in favour of the origin of the Echinoidea from an ancestor in 

 which the apical system was of great importance. Both known genera of 

 Plesiocidaroida are small forms, and they appear to have lived under 

 unfavourable conditions in Triassic lagoons, for the Echinoids with which 

 they are associated are all dwarfed. Hence it seems more natural to 

 dismiss Tiarechinus and Lysechinus as two aberrant genera, in which the 

 test was strengthened by the development of the apical plates. Thus they 

 have no bearing on the character of the ancestral Echinoid. 



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