14:2 C. E. Beecher — Larval forms of Trilobites 



Akt. XXII. — Larval forms of Trilobites from the Lower 

 Helderberg group ; by C. E. Beecheb. (With Plate II.) 



Attention has previously been called to the abundance 

 and perfection of fossils in the siliceous limestones of the 

 Lower Helderberg group, from a limited locality in the Helder- 

 berg Mountains, south of Albany, New York. Proper methods 

 of collecting and preparation yield quantities of the most deli- 

 cate and exquisitely preserved animal remains, consisting 

 chiefly of bryozoa, with numerous brachiopods, ostracodes, 

 spicules of sponges, small corals, lamellibranchs, gastropods, 

 and disarticulated trilobites. Remains of hydrozoa, echino- 

 dermata, annelids, pteropods, and cephalopods, are less fre- 

 quent. 



The conditions of preservation are such that not only are 

 the large 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 brought to light many undescribed 

 species, and has increased the range of known genera, besides 

 affording several genera new to science. Altogether about 

 seventy new species are now known to the writer. 



Among the rarities discovered are the specimens here no- 

 ticed, consisting of Hve larval trilobites referred to the genera 

 Acidasjpis and Phaethonides. They represent early stages of 

 these genera when the animals had no thoracic segments, and 

 when the separation between the cephalon and pygidium was 

 not distinctly marked. 



Notwithstanding the abundance of trilobites in the Paleo- 

 zoic rocks, and the care with which they have been studied 

 and collected, but few are known in the early 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 mate- 

 rial 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, 

 Arethusina, Arionellus, Dalmanites, Hydrocephalus, Sao, 

 and Trinucleus, of which Agnostus, Dalmanites, Hydrocepha- 

 lus (~ Paradoxides), Sao, and Trinucleus, were observed in 

 their larval stages.f 



*Systeme Silurien de la Boheme, i, 1852. 



\ Hydrocephalus appears to be the young of Paradoxides. See Barrande, 

 I. c, and compare Hydrocephalus carens, Pi. 49, H. saturnoides, PL 49, with 

 Paradoxides inflatus, PI. 13, figs. 16, 17, 18, and P. spinosus, PI. 12, figs. 5, 6, 7 



