372 bulletin: museum of comparative zoology. 



area containing the longitudinal striae. Deltoids small, about as 

 long as wide, somewhat flattened at their apices; they appear to be 

 bounded towards the apical end by an angular suture, which is 

 parallel to the lower suture, but this is in all probability merely an 

 ornamentation. Above this line the deltoids are deflected towards 

 the plane of the summit and somewhat hollowed out so as to simulate 

 grooves. The ambulacra are long and narrow, sides parallel ; in side- 

 view occupying the whole height of the body; bordered by the high 

 raised rim of the interambulacral area; the lancet-plate rises in a 

 prominent ridge along the whole length of each ambulacrum. The 

 food-groove is only slightly depressed. There are 28 side-plates in 

 5 mm. The anal opening is many times larger than any of the spira- 

 cles, and is confluent with the two adjacent spiracles. 



Dimensions of three specimens in millimeters. 



Height 6.75 5.5 5.25 



Maximum diameter 7. 5.5 5. 



Length of radial plates 6 . 4 . ? 



Maximum width of radial plates 3.5 2 . 75 2.5 



Length of deltoid plates 1.5 1.5 1 . 75 



Maximum width of deltoid plates .... 1.5 1.5 1.5 



Average number of side-plates in 5 mm. 28 . 28 . 28 . 



Dimensions of the three specimens adjusted to a height of 10 millimeters. 



Height 10. 10. 10. 



Maximum diameter 10 . 4 10 . 9.5 



Length of radial plates 8.9 7.3 ? 



Maximum width of radial plates 5.2 5. 4.8 



Length of deltoid plates 2.2 2.7 3.3 



Maximum width of deltoid plates 2.2 2.7 2.86 



Formation and locality. — The specimens are from the Madison 

 limestone of Old Baldy and of Squaw Creek, Montana, collected by 

 Dr. W. P. Haynes and the writer. 



This species differs from Schizoblastus lotohlastus (Suboarboniferous 

 of Arizona and Idaho) in having a base not concave and in being 

 globose, while in Schizoblastus lotoblastus the form is subelliptical in 

 outline. Moreover the latter form is distinctly pentalobate at the 

 base. 



