NIAGAEA GEOtrP*, 



235 



alike in form, in each pair. In the higher or lateral pairs, although the individuals are some- 

 times placed nearly in a line parallel to each other, one is always a little higher, and sometimes 

 nearly, or directly, above the other. The higher one has a nearly straight base, usually not much 

 elevated, and sometimes coincident with the margins of the plate : on its upper side it is arched 

 or angular, with an elevated border, making the entire outline triangular or sub-reniform. The 

 bars crossing this aperture are consequently longer in the centre, and gradually diminish 

 towards each extremity. The lower aperture is often quite oval, sometimes elongated or almost 

 crescent-form, but always bounded on all sides by an equally elevated rim. When we examine 

 the basal pair, the relative position of the two apertures is reversed, and the triangular or 

 sub-reniform one, which in the higher pairs is above, is here placed below the oval one. Thus 

 whatever may be the form, or the relative position of the two forms in the upper pairs, it is 

 always reversed in the lower pair, or the relative position of the two in the lower pair is 

 reversed in the upper pairs. Nor is this all : the pectinated space in the oval aperture, or the 

 lower one of the upper pairs and the higher one of the lower pair, is always deeper below 

 the surface of the plate ; and the protecting rim or margin is stronger, than in the angular or 

 higher aperture of the upper pairs, where the bars reach nearly or quite as high as the surface 

 of the plate, and the rim on the upper side is thin. From this cause, the upper ones have often 

 suffered from abrasion, and are frequently obliterated, while the lower or oval ones still remain 

 nearly perfect. In many such instances they may be overlooked, since they appear much like 

 cystalline striae ; or like the striae upon the interior of plates of Caryocrinus when worn down 

 from the external surface. 



Such is the character of these organs, as existing in several very distinct species which have 

 been examined. They have doubtless a greater variety of form and mode of development than 

 here described, but it is believed that the important characteristics are given. 



Many, if not all the cystideans, were attached, like the crinoids, by a column or pedicle, 

 which is usually very short, rapidly diminishing to the base, and the joints growing thicker or 

 becoming anchylosed as they recede from the animal. These columns or pedicles present con- 

 siderable variety in their external appearance, the joints being often expanded on their lower 

 margin, and overlapping the upper part of the one below, giving a reverse cup form, as if 

 the lower were inserted into the next succeeding joints, very much like the internal casts of 

 CoRNULiTES when placed with its larger extremity upwards. 



The Cystidece, both in this country and in Europe, are known to have commenced their 

 existence in the Lower Silurian period (as shown in Pal. N. Y., Vol. 1, p. 87), where we 

 have but a single American species yet known. In the Upper Silurian period we have several 

 (at least four) well ascertained species* ; while in the Devonian period we have no well 



* One of these has been for a long time well known through the indefatigable zeal of Mr. Gebhard of Schoharie, 

 who had thoroughly studied its structure previous to 1840, when it was published by Mr. Conrad as Lepocrinus 

 ILepadocrinus'] gebhardii, in the Annual Report on the Palaeontology of New-York for that year, without any de- 

 tailed description. This species is so closely allied to Pseudocrinites, Pearce, that it may be necessary to cancel 

 one of the names. The name has allusion to the short pedicle and form, altogether like Anatifa. This one will be 

 published in the succeeding volume. 



