Larval forms of trilobites from the lower Helderberg group

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1893
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Abstract
Trilobites ART.XXII.-La}'1)al forms of Trilobites front the Lower Ilelderbel'g gJ'otlP" by O. E. BEECHER.(With Plate II.)ATTENTION has previously been called to the abundance and perfection of fossils in the siliceous limestones of the Lower Helderberg gl'OUP, from a limited locality in the Helderberg Mountains, south of Albany, New Y (lrk.Proper methods of collecting and preparation yield quantities of the most delicate and exquisitely preserved animal remains, consisting chiefly of bryozoa, with numerous bmchiopods, ostracodes, spicules of sponges, small corals, lamellibranchs, gastropods, and disat•ticula~ed trilobites.Remains of hydrozoa, echinodermata, annehds, pteropods, and cephalopods, are less frequent.The conditions of preservation are such that not only are the largc and strong species preserved, but also the smallest, and the young of many in all stages of growth.As would naturally be inferred, the study of this fauna, based upon such complete material, has bl'Ought to light many undescribed species, and has increased tho range of known genet'a, besides affording several genera new to science.Altogether about seventy new species are now known to the wl'iter, Among the rarities discovel'€d are the specimens here noticed, consisting of five lat'val tt'ilobites referred to the genera Acidaspi8 and Phaetlwnide8, They represent early stages of these genera when the animals had no thoracic segments, and when the separation between the cephal on and pygidium was not distinctly mal'ked.Notwithstanding the abundance of trilobites in tIle Paleozoic rocks, and the care with which they have been studied and collected, but few are known in the earl.Y adolescent state, and extremely few in the still earlier larval stages.This paucity of larval specimens seems to be due chiefly to the sediments, which, In general, have not been favorable for the preservation of such organisms.Heretofore, the best material has come from fine shales and slates representing original muds.The most complete developmental series known are the classic ones of Barrande,* representing Aeglina, Agnostus, A? 'et/w8ina, Al•ionellu8, DaZmanite8, Hydrocephalu8, Sao, and Tl'inucte1l8, of which Agno8tlt8, Dalmanite8, Hydroceplw-l1t8 (= Paradoxides), Sao, and l'rinucleu8, were observed in their larval stages.t
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Authors C. E. Beecher
Journal american journal of science
Year 1893
DOI
10.2475/ajs.s3-46.272.142
URL
https://doi.org/10.2475/ajs.s3-46.272.142
Keywords Keywords not found

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