UPPER SILURIAN. 243 



475 a represents a cast of the interior of Spirifer arenosus. There are 

 also many other species of Brachiopods, and a number of Lamelli- 

 branchs, Gasteropods, and Gephalopods. Among the Gasteropods, the 

 shells of Platyceras are in some places very numerous ; they are a thin 

 shell of a floating Mollusk, related to the delicate Ianthina of modern 

 seas ; as stated by Hall, they often occur in the Maryland beds, in 

 groups, as if drifted together by the winds or gentle currents. Cri- 

 noids are rare fossils in New York, but common in Maryland. No 

 Fishes have yet been found in the beds. 



The Crinoids in Maryland include a number of fine species of the genera Mariacri- 

 nus, Edriocrinus, and others, besides three species of Cystideans, and among them one 

 of the peculiar genus Anomalocystites (allied to Fig. 458). The rock in some places con- 

 tains a wonderful profusion of shells, although the number of species is small. 



Rensselaeria oroides, Spirifer arenosus, together with the Cauda-yalli fucoid (Fig. 484, 

 p. 255) and three species of Chonetes, occur in the upper 500 feet of the Gasps' lime- 

 stone, as determined by Billings, associated with Favosites Gothlandica Lam., F. 

 basaltica Goldf., F. cervicornis De Blainville, two species of Zaphrentis, Strophomcna 

 rhomboid alis, S. Bechi, S.perplana Con., Leptoccdia concava H., L. flabellit.es H., Eato- 

 nia peculiaris H., Atrypa reticularis, Meristella lams H., species of Modiolopsis, Avicula, 

 Murchisonia, Loxonema, Orthoceras, Phacojis, Proetus, also Dalmanites pleuroptyx, 

 etc. The fucoid extends down 800 feet, and is abundant. At Parlin Pond, in northern 

 Maine, there occur Rensselaeria ovoides, Leptocadia flnbellites, Spirifer arrectus EL, S. 

 pyxidatus H., Strophomena (Hemipronites) maynifica H., Rhynchonella oblata H., Orthis 

 musculosa H., Dalmanites pleuroptyx, species of Chonetes, Modiolopsis, Cyrtodonta, Avic- 

 ula, Murchisonia, Platyostoma, Orthoceras. 



The ribs of some Oriskany Spirifers have a peculiarity observed in only one other 

 American Silurian species (of the Niagara epoch), but in Europe not known before the 

 Devonian age, — which is, that they subdivide dichotomously, instead of being simple. 

 The shell, in the genus Rensselaeria Hall, contains a loop-like arm-support, a little like 

 that in Terebratula, but it is only curved, instead of bent, and has a spade-shaped ter- 

 mination. 



HI. General Observations. 



The Oriskany sandstone is another of the arenaceous rocks ranging 

 from central New York to the southwest, along the Appalachian 

 region, and thus serving to define the old Appalachian sand-reef. As 

 in other cases, the rock thickens on going from New York to the 

 southwestward. The fossils and the distribution of the formation over 

 the State of New York, seem to point to the existence at this epoch 

 of inland waters opening into the ocean to the southeast, — as might 

 have existed if the Green Mountain region (as before in the Upper 

 Silurian era) were out of water, and if also the Archaean of northern 

 New Jersey (see p. 150), the proper continuation of the Green Moun- 

 tains, were an island or reef in the sea. The muddy and sandy bot- 

 tom of the bay would have given the shells a fit place for growth. To 

 the south, as the fossils in Maryland and beyond show, the accumula- 

 tions were those of an open bay or coast, where there were at least 



