INVERTEBRATE PxVLiEONTOLOGY. 5 



ECHINODERMATA. 



ECHINOIDEA. 



SPATANGIDiE. 

 Genus HEMIASTER, Desor. 



Synon. — Spatangus, Micrastcr, &c. (sp.), of several authors. 



Semiaster, Desor (1847), Cat. Rais., 121; ami (1858), Synop. fich. Foss., 367. 



Etym. — >;/j.i, a half; aorijp, a star. 

 Examp. — Hemiaster phrynus, Desor. 



Body small, short, and depressed, truncated posteriorly ; provided with 

 a more or less angular peripetal fasciole; without lateral, subanal, or marginal 

 fascioles. Ambulacra well defined, a little concave, and diverging; anterior 

 groove not very profound, often narrower than the lateral or posterior ambu- 

 lacra. Peristome bilabiate. Apical disk very compact, and showing distinctly 

 the four genital pores. 



As has been observed by Professor Desor, in his " Synopsis des Echinides 

 fossiles,'' this genus includes two sections, one of which, the typical Hemi- 

 asters, is characterized by having much shorter posterior than anterior 

 ambulacra, while in the other the anterior and posterior ambulacra are of 

 nearly equal length. These two subgroups would seem, upon a hasty exami- 

 nation, to be sufficiently distinct to rank as genera; but on comparison of a 

 large number of species, the transition from one to the other is discovered 

 to be so gradual that it has not been found practicable to separate them into 

 distinct genera. 



So far as known, the genus Hemiaster seems to have been introduced 

 during the deposition of the ''Lower Green-sand" of English geologists. It 

 ranges through the other Cretaceous deposits, during the deposition of which 

 it attained its greatest development. It is also represented by a number of 

 species through the various Tertiary rocks, even down to the Miocene, but is 

 not known among the recent Echinoidea, 



Hemiaster II u in i> hrcysauus, M. & H. 



Plato 10, figs. 1, a, b, c, d, e,f, g. 

 Hemiaster '! Humphreysanw, Meek aud Hayden (May, 1857), Proceed. Acad. Nat. Sci. Philad., 147. 



General form broad cordate, oval, or subcircular; greatest elevation near 

 the posterior side, declining very slightly toward the front, broadest a little in 

 advance of the middle, and flattened below; oral aperture small, transversely 



