﻿HEMIASTER. 



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subanal. We find sometimes in the same genus more fascioles than one ; thus the 

 subanal and peripetal are frequently associated together. 



This family contains many genera, none of which are found in rocks older than the 

 Cretaceous formations ; the species increase in number in the Tertiary beds, and attain 

 their greatest development in our present seas. In the Cretaceous rocks we find the 

 extinct genera 



Hemiaster, Desor. Enallaster, d'Orbigny. 



Epiaster, d'Orbigny. Heteraster, d'Orbigny. 



Micraster. d'Orbigny. Echinospatagus, Breynius. 



The new genus Paleopneustes, Al. Agassiz, proposed for a species brought from 

 Barbadoes by the Hassler expedition, appears to furnish an interesting link between the 

 EchinocoridjE and SPATANGiDiE. In its general form it resembles Echinocorys 

 vulgaris, its anteal sulcus is rudimentary, and it has structural affinities with the anterior 

 single area of that Urchin. The other ambulacra are subpetaloidal ; and the peristome 

 bilabiate with well-developed lips. 



Genus — Hemiaster, Desor, 1847. 



Urchins with a short, elevated, inflated, or cordiform test. The ambulacral summit 

 in general excentral and posterior. The pairs of ambulacra petaloidal, unequal in length, 

 and lodged in depressions of the surface ; poriferous zones large and equal in the same 

 ambulacra, the pores elongated and placed close together. The single ambulacrum 

 lodged in a long, shallow, anteal sulcus ; the poriferous zones are very narrow and com- 

 posed of small round pores, sparsely disposed in oblique, widely separate, simple pairs. 



The fasciole single, peripetalous, and circumscribing the ambulacra. 



The apical disc small and compact, four perforated genital plates, and five very small 

 oculars. 



Peristome bilabiate, very excentral, opening at the anterior fourth part of the base. 

 Periprocte opening high up on the posterior border, which is in general flat, and 

 obliquely truncated. 



Hemiaster differs from Micraster in having a single peripetalous fasciole and no anal 

 fasciole ; the test likewise is in general shorter, more inflated, and the posterior pair of 

 ambulacra are much shorter than the anterior pair. Hemiaster differs from Periaster in 

 having only a peripetalous fasciole, the latter having both peripetalous and lateral 

 fascioles. 



