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 milled ring ; and they have the surface 

 sculptured with fine, longitudinal lines. 



The Holectypi are distinguished from the Ec/iinoconi by having a larger mouth 

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

 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 

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

 from the Hyboclypi. 



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. = \0 e Etage Bajocien, D'Orbigny. 



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



Jacob, a Melle de Echinitis Wagricis, tab. 1, fig. 2, 1/18. 



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



Bruckner, Merkwiirdigkeiten der Landschaft. Basel, tab. 22, 



figs, g, H, 1748-63. 

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



(Egelsteen tienb and 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, Systema 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 Germaniae, 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. 



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



— depressus. Grateloup, Memoire sur les oursins Fossiles (Dax), p. 56, No. 9, 



1836. 



