86 GEOLOGICAL MEMOIRS. 



distinguished, first, by the unusually large dimensions of the stem ; 

 secondly, by the very large and projecting ovarial aperture, which 

 is situated in the lower instead of, as in other cases, the upper half 

 of the cup ; thirdly, by the elongated form of the mouth ; fourthly, 

 by the very strikingly prominent rhombic striae passing from one 

 plate to another, and rendering the actual line of demarcation be- 

 tween the plates generally very difficult to find ; and lastly, by a 

 singular and finely striated basal plate, and a similar finely striated 

 segment of a plate placed diametrically opposite, between the ova- 

 rial orifice and the mouth. 



The stem is very slender at its further extremity, and is provided 

 with articulations whose length is three or four times greater than 

 their diameter. Towards the cup the diameter, however, increases, 

 the articulations approach one another and become rings, and at 

 length when they reach the basal plate and pass into it, this diameter 

 is as much as one-third of the Avhole diameter of the cup (Voll- 

 borth. Bulletin de I'Acad. de Petersb. x. tab. 2). 



The base of the cup into which the stem passes is a nearly perfect 

 square, which may become changed into a rhomb, the angles of 

 which are blunted by the compression of the entire form. The 

 basal plates are deeply depressed near where the stem is attached. 



Four basal plates surround the depressed square in such a manner 

 that each plate encloses one angle of the square, and its edges reach 

 the middle points of the two enclosing sides. Three of the plates 

 are teriinnated by similar triangles having the vertical angle directed 

 upwards, but the fourth, in the direction of the ovarial orifice, has 

 this vertical angle truncated. Hence it becomes possible that these 

 four plates may support five parabasalia (corresponding to lateral 

 plates), namely four alternating with and occupying the spaces be- 

 tween the triangular ones, and the fifth placed on the truncated 

 angle towards the ovarial opening. A group of five parabasalia of 

 the second order is placed alternating with and above these, and 

 lastly, another group of five of smaller size and of the third order 

 surround the mouth. All these plates are covered with very pro- 

 minent striae, which project like ridges and are at right angles to 

 the edges of each plate, continuing without interruption from one 

 plate to another, as in most of the Crinoidea and Cystidea. Only 

 three ridges proceed to each edge, and two smaller ones pass into 

 these to form a prominent triangle where three plates meet. Since 

 therefore every plate is properly a hexagon, we have rhombs pro- 

 ceeding from each of the six edges, their longer diameters and acute 

 angles meeting in the central points of two adjacent plates. Striae 

 of growth parallel to the edges of the plates fill up the space be- 

 tween the other striae and at right angles to them, but are by no 

 means so prominent. 



It is yet more remarkable, but still common to all the individuals 

 of this species, that one of the basal plates exhibits much more deli- 

 cate perpendicular striae than the rest, and that these fine striae pro- 

 ceed to the adjacent plate. Instead of three of such striae, also, 

 there are no less than ten, and they appear at first sight to form two 



