SKA-UKC'HIXS. 449 



Hubgenus 2. Mkgapyous nov. {Pi/goster lesti', Poniel). 



Pygaster with periproct pyriform, and coiistricterl towai'ds the 

 apex. Fifth genital phite present, ov replaced by several small 

 plates. Tubercles with shallow scrobicules and irregular in 

 their introduction. 



Subgenotype. M. unihrella (a.uctt.). 



Middle and Upper Oolitic. 



Subgenus 3. Macropygus Cotteau. 



Pi/gastcr with the periproct and apical system as in Megapygus. 

 Posterior mai'gin sti-ongly truncated. Tubercles with large, deep 

 scrobicules; regular in their inti'oduction. 



Subgenotype. M. trnncatus Agassiz. 



Loirer Oolitic to Loiver Cretaceous. 



Subfamily 2. P i L E i N .v.. 



Pygasteridse with the periproct midway between the apex and 

 the ambitus. Madreporite very large, occupying the centre of 

 the apical system. 



r.enus 1. PiLEUs Desor. 



Pileinse with the ambulacral pore-pairs biserial adapically. 

 Periproct very slightly oblique. Fifth genital plate small, 

 imperforate. 



Genotype. P. piletts Agassiz. 



Corallimi. 



Genus 2. Axorthopygus Cotteau. 



Pileina^ with the ambulacral pore-pairs uniserial throughout. 

 Periproct markedly oblique. Apical system ethmolysian. 

 Tubercles of intera,mbulaci*a in sloping lines on both sides of the 

 central series. 



Genotype. A. orbicularis Grateloup. 



Loioer andj Middle Cretaceous. 



Family II. DISC0IDIIJ3^. 



Holectypoida with a circular or posteriorly elongated ambital 

 outline. Depressed or elevated, conical, adapically; flat or 

 slightly concave adorally. Peristome of moderate size, with 

 strong perignathic processes and well-developed ridges. Bran- 

 chial incisions well marked. Periproct of variable size, marginal 

 or adoral in position. Ambulacral plates primaries to the 

 ambitus, compound there and adorally. Pores usually equal and 

 circular. Interambulacra broad, with many vertical rows of 

 tubercles, often supplemented by hypertrophied granules. 

 Granulation usually transversely linear. Tubercles much larger 

 on the adoral than on the adapical surface ; arrangement as in 



Proc. Zool. Soc— 1912, No. XXIX. 29 



