Weno and Pawpaw Formations 1J7 



the diameter of a pore apart. These are situated in the antero-median 

 quadrant of a quadrate or trapezoidal ambulacral plate which bears two 

 or three large tubercles. The posterior zone consists of about 32 pairs, 

 each having an anterior oval pore and a posterior wedge-shaped slit nar- 

 rowest at the forward end. 



Postero-lateral ambulacra: Short, rather wide and bulging in the cen- 

 ter, axes straight diverging at an angle of about 115 degrees from each 

 other. The anterior zone has about 12 pairs of short slits; the posterior 

 about 14 pairs of nearly equal longer slightly oblique slits. All of these 

 pores decrease in size toward each end of the zones. 



Apical system: Four genital plates, the right anterior one being the 

 elongated madreporite, the posterior pair slightly farther apart than the 

 anterior pair ; four perforated oculars. 



Peristome: Transversely oval with a slightly emarginate lip. The lip 

 posteriorly is elevated into a blunt high tip or carina. 



Periproct: In shape an oval, transversely situated but with the rounded 

 inferior margin slightly bulged downward; each diameter (on the type) 

 about 2.8 mm., situated high on posterior truncated surface two-thirds the 

 way up. 



Aside from generic characters the following differences will assist in 

 separating H. riovistae n. sp. from Enallaster wenoensis n. sp. E. weno- 

 ensis has the apical system farther forward; the antero-laterals diverge 

 at a slightly greater angle ; the anterior ambital notch is deeper and more 

 sharply incised; the form is broader, less elongate, more nearly circular, 

 distinctly less flat and more constricted posteriorly. The area in which 

 the periproct lies is not merely truncate, but is sharply excavated giving 

 a narrow vertical groove, while in H. riovistae the posterior end of the 

 test is broadly truncate and the excavation is broad and shallow; and the 

 pores of the anterior unpaired ambulacrum are in part alternating long 

 and short pairs, instead of being all similar and separated by a tubercle 

 as in H. riovistae. The more posterior position of the apical system of 

 H. riovistae results in its having a longer anterior sulcus, it being inter- 

 mediate in this respect between E. wenoensis and H. longisulcus. The an- 

 tero-laterals are in many individuals which are here referred to E. weno- 

 ensis much more sunken than in H. riovistae. The only other described 

 species with which H. riovistae might be confused is E. bravoensis which 

 has long and short pore pairs in the anterior unpaired ambulacrum instead 

 of having all the pore pairs similar. H. longisulcus (Adkins and Winton) 

 has these pore pairs similar, but it is at once distinguished by its lower, 

 more elongate form, and by the unusually long anterior sulcus and the con- 

 sequent posterior position of the apical system. 



