l86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



Shape. — Medium size, oval, inflated, greatest width and height 

 posterior to center ; margin gently rounded except for slight posterior 

 truncation, peristome depressed. 



Apical system. — Anterior, monobasal, madreporite inflated. 



Ambulacra. — Petals narrow, petal III shorter than others, II and 

 IV extending over two-thirds distance to margin ; petals V and I 

 over one-half distance; interporiferous zones over twice width porif- 

 erous zone; petals closing distally; poriferous zones depressed, of 

 unequal length : one more pore pair in right zone of petal III, two 

 more in posterior zones of petals II and IV, six more in outside zones 

 of petals I and V ; this character variable with specimens ; pores con- 

 jugate, outer pore elongate transversely, inner smaller and round. 



Adoral interamhiilacra. — Single plate (text fig. 152) at peristome, 

 preceded by two alternating plates, one much larger than the other 

 in interambulacrum 3, smaller plate adjacent to ambulacrum III ; in 

 interambulacra i and 4 smaller plate adjacent to ambulacra I and V 

 respectively ; in interambulacrum 5 adjacent to ambulacrum I ; plates 

 preceding these regularly alternating, of approximately equal size. 



Periproct. — Marginal to slightly supramarginal ; transverse with 

 slight groove at adoral side of opening. 



Peristome. — Anterior, depressed, pentagonal. 



Floscelle. — Bourrelets well developed (pi. 28, fig. 8) ; vertically 

 sided. Phyllodes (text fig. 151) widened single pored, two series 

 in each half -ambulacrum : seven or eight pores in each outer series, 

 four to five in each inner series ; buccal pores, sphaeridia present. 



Tiiberculafion. — Adorally, tubercles larger ; naked median band 

 in interambulacrum 5, short length at ambulacrum III. 



Occurrence. — Middle Eocene of France. 



Location of type specimen. — According to Lambert and Jeannet 

 (1928, p. 126) the type is in the ficole National Superieure des Mines. 



Remarks. — Lambert erected this genus for all the Tertiary species 

 of Pygorhynchus. Later, he (Lambert and Thiery, 1921, p. 370) 

 placed his genus in synonymy with Rhyncholampas. Mortensen 

 (1948, p. 203) considered it, along with Rhyncholampas, as a syno- 

 nym of Cassidulus. Cooke (1959, p. 59) maintained Plagiopygus as 

 a subgenus of Cassidulus, distinguishing it from the latter by its 

 more terminal periproct, and more inflated posterior extremity. 



The type species of Plagiopygus is quite similar to the type species 

 of Rhyncholampas and certainly congeneric with it. Both species 

 have approximately the same shape, similar petals, although slightly 

 more lanceolate in R. pacificus, transverse periproct and peristome. 



