260 



HOLECTYPUS. 



The internal moulds of Holectypus want those depressions occasioned by ribs 

 projecting from the inner walls of the test, which so well characterise Discoidea. 



The spines are short, with a smooth head, and miUed ring ; and they have the surface 

 sculptured with fine, longitudinal lines. 



The Holectypi are distinguished from the EcJiinoconi by having a larger mouth 

 and vent, a concave base, and a less-elevated dorsal surface; and from the Discoidece 

 in having tumid sides, a larger mouth and vent, and the absence of ribs from the internal 

 wall of the test. 



The small crenulated tubercles, and basal vent, with the absence of any aperture 

 in the upper surface of the inter-ambulacrum, distinguishes the Holectypi from the 

 Fygasters ; and the want of a longitudinal valley in the inter-ambulacrum separates them 

 from the Hyhodypi. 



The Holectypi are abundant chiefly in the Oolitic rocks ; two of the species are found 

 in the Neocomian and the Chalk. They make their first appearance in the Inferior 

 Oolite, in the zone of the Ammonites Parkinsoni, Sow. 



A. Species from the Inferior Oolite. ■= 10* Etaye Bajocien, D'Orbigny. 

 Holectypus depressus, Leshe. PL XVIII, fig. 1 a, b, c, d, e, f, y, h, i. 



Jacob, a Melle de Echinitis Wagricis, tab. 1, fig. 2, 1718. 



Kundman, Rariora Naturae, tab. 5, fig. 12, 1737. 



Briickuer, Merkwiirdigkeitea der Landscbaft. Basel, tab. 22, 



figs. G, H, 1748-63. 

 Van Phelsum, Brief, de Gewelvslekken of Zee-egelen, p. 31, No. 16 



(Egelsteen tienhand plattop), 1774. 

 Knorr, Petrefactions, vol, ii, tab. E, ii, figs. 6, 7, 1775-78. 

 Favanne, pi. 67, figs. 1, 2. 

 EcHiNiTES DEPRESSUS. Leske, Additamenta ad Kleinii Echinodermata, p. 164, pi. 41, 



figs. 5, 6, 1768. 

 Echinus depressus. Linne, Systeraa Naturae, Gmelin, p. 3182. 



Galerites depressus. Lamarck, Animaux sans Vertebres, torn, iii, p. 21, 1816. 



— — Deslongchamps, Enc, torn, ii, p. 432; Encyl. Method., tab. 152, 



figs. 7, 8. 



— — Defrance, Diet, des Sciences Naturelles, tome xviii, p. 86, 1820. 

 EcHiNiTES ORiFiciATUS. Schlotheim, Jahrb., 1813; Petrefactenkunde, p. 317, 1822. 

 Galerites depressus. Goldfuss, Petrefacta Germanise, Band i, p. 129, tabl. 41, fig. 3, 



1826. 



— — Phillips, Geology of Yorkshire, tab. 7, fig. 4, 1829. 



— — De Blainville, Zoophytologie, p. 204, 1830. 



— depressa. Leonhard and Bronn, Jahrb., 1834, p. 135. 



— — Desmoulius, Etudes sur les Echinides, p. 254, No. 6, 1835. 



— DEPRESSUS. Grateloup, M^moire sur les oursins Fossiles (Dax), p. 56, No. 9, 



1836. 



