HEMICIDARIS. G9 



Genus— UmilClDAmS, Agassiz. 1840. 



CiDARiTES (pars), Lamarch. 1816. 

 CiDARiTES (pars), Goldfuss. 1829. 

 DiADEMA (pars), Besmoulins. 1835. 



The genus Hemicidaris was established by Agassiz,* to receive certain urchins which 

 Lamarck and Goldfuss had included in the genus Cidarites, and Desmoulins had placed 

 among his Diademas. The dismemberment of these forms from the genera with which 

 they had been associated., was an important progressive step in stratigraphical palaeon- 

 tology, as they form extinct types of the Echinoidea, which, up to the present time, 

 have only been found in the Oolitic, Cretaceous, and Nummulitic Formations. 



The species of this genus exhibit a group of characters which are easily recognised, 

 even when portions of the test only are preserved. 



If the reader will please to compare Plates I and II with Plates III and IV of this 

 work, he will discover at a glance how widely different the general /ac^'e^s and structure of 

 the test in Hemicidaris is from that of the true Cidaris. 



The Hemicidaris, in general, have the test thick, of a medium size, more or less sub- 

 globose, and generally flattened at the base ; the altitude being greater in proportion to the 

 latitude than in either the Cidarid^, Diademad^, or Echinid^e. The distinctive character 

 of the genus lies in the structure of the ambulacral areas, which are narrow, and more or less 

 flexuous. At the enlarged base of each area (PL IV, fig. 1 b, c, d) there are three or four 

 pairs of mammillated tubercles, which occupy the lower fourth part of the area. These 

 semi-tubercles are smaller in size, but identical in structure, with the primary tubercles of the 

 inter-ambulacral areas, and, like them, they gradually increase in magnitude from below 

 upwards. The upper three fourths of the area has two marginal rows of minute perforated 

 tubercles, which contrast strongly with the large semi-tubercles they immediately succeed. 

 Between the minute marginal tubercles there is a narrow miliary zone, containing two or 

 more rows of small, close-set granules. 



The poriferous zones are narrow ; the pores are unigeminal, and approximated on the 

 sides, but near the peristome they become bigeminal and trigeminal, according as the 

 space in that region is more or less augmented. 



The inter-ambulacral areas are wide, with two rows of primary tubercles, from four to 

 eight in each row. They have large, prominent bosses, with deeply crenulated summits, 

 especially those near the equator of the test ; the areolas are wide, and mostly confluent 

 above and below, so that the scrobicular circle of granules is generally incomplete. In 

 some species the tubercles increase and diminish gradually in the area ; in others, 



* 'Description des Ecbinodermes Fossiles de la Suisse,' part ii, p. 42. 



