298 



rostrum wlien such 



ECHINODERMATA— ECHINOZOA 



PHYLüM IV 



Amhulacra flush, apetalous, sometimes discontinuous ; 

 pores Single or slit-Uke. 



Pourtalesia, Spatagocystis 

 and Echinocrepis A. Agassiz. 

 Recent. 



Order 5. PLESIOCIDA- 

 ROIDA Duncan. 



Test regulär, endocyclic. 

 Genitals largely covering the 

 dorsal surface. Two columns of 

 low simple plates in each am- 

 bulacral area and three columns 

 of plates in each interamhulacral 

 area. Plates not imbricate. 

 Primordial interamhulacral 

 plates in basicoronal row. Base 

 of Corona not resorhed. Oculars 

 small, strongly exsert hy the con- 

 tact of large genitals. Peri- 

 proct central, structure unknown. 

 Peristome central, structure un- 

 known. Lantern and perigna- 

 thic girdle unknown. 



Family 1. Tiarechinidae 



Zittel. 



The Single primordial plate 

 of the interambulacra followed 

 hy three elongated plates only, 

 one on either side of a narrow 

 median plate. Trias. 

 Tiarechinus Neumayr (Fig. 426). The test of this unique genus is very 



small, flat actinally, and sub-hemispherical dorsally. Below the ambitus and 



actinally the Ornament consists of a piain 



primary tubercle to each plate ; elsewhere the 



test is coarsely granulär, including the very 



large apical System. The solitary species, 



T. princeps (Laube), occurs in the Trias of St. 



Cassian, Tyrol. 



Our knowledge of this genus and family is 



based largely on Lov6n's study of a single 



minute specimen. As it has three columns 



of plates in an interamhulacral area, it is 



considered a further remove from the primi- 

 tive than those types with two columns. Three plates, representing three 



columns, immediately succeeding the primordial interamhulacral plate, is a 



unique character in Echini. 



Fig. 425. 



Macropneustes meneghinii Desor. Eocene ; Monte Spiado, near 

 Vicenza. 



Fio. 426. 



Tiarechinus %ii-inceps (Laube). Upper Trias ; 

 St. Cassian, Tyrol. Ventral and lateral 

 aspects, mncli enlarged (after Loven). 



