NO. 8 EDRIOASTEROIDEA BASSLER 5 



Genotype. — Agelacrinus cincinnatiensis Roemer, 1851. Ordovician 

 (Maysville) of Ohio, etc. Other species of Isorophus are Agela- 

 crinus austinl Foerste, 1914, A. faberi Miller, 1894, A. warrenensis 

 James, 1883, and A. holbrooki James, 1878. 



ISOROPHUSELLA, n. gen. 



Like Isorophus but ambulacra i, 2, and 3 directed to the left and 4 

 and 5 to the right, and the interambulacral plates strongly imbricating. 



Genotype. — Lcbetodiscus inconditiis Raymond, 191 5. Ordovician 

 (Trenton) of Ontario, Canada. 



ISOROPHUSELLA INCONDITUS (Raymond) 

 Plate I, fig. II 

 Lcbetodiscus inconditiis Raymond, Ottawa Nat., vol. 29, pi. i, fig. i, 1915. 



Theca a thin, slightly convex, circular disk, attached to or resting 

 upon a limestone bed by its entire aboral surface, about 20 mm in 

 diameter, exhibiting on its oral face a broad peripheral band of minute, 

 closely imbricating plates, five ambulacral areas of which the oral por- 

 tion is composed of many small irregular, angular plates, and each 

 ambulacrum has two rows of covering plates alternating with each 

 other and frequently developing additional small accessory plates 

 along the midline between them. Ambulacra short, rather broad, 

 three (1-3) curving to the left and two (4, 5) to the right. Inter- 

 ambulacral areas of larger, narrow, strongly imbricating plates. Anal 

 area rather distinct, consisting of 10 elongate triangular smooth plates 

 in one circle meeting at the center but not rising above the general 

 surface. 



Although very similar to species of Isorophus, this interesting form 

 is readily distinguished by the dififerent arrangement of the ambulacra. 



Occurrence. — Trenton limestone, Hull, Quebec. 



Plesiotype. — U.S.N.M. no. S.3871, Springer Coll. 



STREPTASTER Hall, 1872 



Theca as in Carneyclla, except that the ambulacra all revolve 

 strongly toward the left and are composed of highly elevated plates. 

 Interambulacrals are small, polygonal, mosaic plates, 0.5 to i.o mm in 

 diameter. 



Genotype. — Agelacrinus vorticellatus Hall, 1866. Ordovician 

 (Maysville) of Ohio. S. rcversatus Foerste, 1914, and .9. septem- 

 hrachiatus Miller and Dyer, 1878, are other species of Streptaster. 



