448 MR. HERBERT L. HAWKINS ON 



Ambulacra straight, narrow, similar, and simple, with usually 

 lari^er poclial pores on the adapical than on the adoral surface. 

 Towards the peristome the pore-pairs may become grouped 

 into arcs of three. Ambulacra! plates small, simple or compound, 

 the latter always derived from three original priraary plates. 

 Interambulacra broad ; the plates usually concentric in arrange- 

 ment, rarely bent along their median line. Tubercles always in 

 recognizably vertical series. Apical system variable ; madreporite 

 usually large. Radioles short, and longitudinally striated. 

 Liasslc to Uppermost Cretaceous. 



Family I. PYGASTERID^. 



Holectypoida with a circular or posteriorly truncated a.mbital 

 outline. Bluntly conical adapically, concave adorally. Peristome 

 laro-e, with strong pei-ignathic processes and feeble ridges. 

 Jaw-structure insuiScieutly known for diagnosis. Branchial 

 incisions deep. Periproct large, always on the adapical surface, 

 often oblique. Ambulacra slightly tumid, composed of primaries 

 to a point about midway between the ambitus and the peristome. 

 Outer members of pore-pairs often transversely elongated (to a 

 slio-ht degree only) on the adapical surface. Interambulacra broad, 

 paucituberculate (for an Irregular Echinoid), the admedian 

 tubercles being in concentric series, the adradial oblique. 

 Grranulation irregular, faintly scrobicular. Apical system with 

 four perforated genital plates ; the fifth being usually represented 

 by several smalt plates. Ocular plates small and similar. No 

 internal buttresses to the test. Radioles short, longitudinally 

 striated. 



Liasslc to Lower Cretaceous. 



Subfamily 1 . P y G A s T B R i N ^. 



Pygasteridpe with the periproct in contact with the apical 

 system. Ambulacral pore-pairs uniserial except near the peri- 

 stome. 



Genus 1. Pygaster Agassiz. 



With the characters of the subfamily. 

 Genotype, P. semisulcatus Phillips. 



Subgenus 1. Pygaster sens. str. [ = Plesiechiniis Pomel). 



Piigaster with the peripi-oct inside, as well as outside, the 

 apical system, and with its greatest width in the adapical part. 

 No posterior genital plate, the remaining plates of the system 

 being arranged transversely. Tubercles witli shallow scrobicules, 

 regular in their introduction. 



Subgenotype. P. semisulcatus Phillips. 



Liasslc to Middle Oolitic. 



