150 TRANS. ST. LOUIS ACAD. SCIENCE. 



small three-cornered segments of about y\) of an inch in thick- 

 ness, have unequal margins, of which the largest, joining with 

 the fork piece, is convex, while the opposite one, joining with 

 the lancet piece, is concave. They close up the furrow on each 

 side of the lancet piece, and according to its dimensions they 

 will vary in size like the rest of the pieces forming the calyx. 

 They are inserted edgeways, and have, on each side of the margin 

 which connects with the fork piece, a minute semilunar sulcus,, 

 which with its opposite fellow forms a poral opening (sec PI. a^ 

 Fig. 7)- 



c. The zigzag plicated integument. This may be regarded as- 

 a band, which is transversely plicated and covers the whole am. 

 bulacral field ; it was probably of an elastic texture during the 

 lifetime of the animal. It commences at the apex of the ambula- 

 cx'al field running in a zigzag from the lateral marg n to the me- 

 dian line, so that the poral openings are always phiced between 

 two returning folds, which are flattened here to form a sort of 

 articulating surface for the pinnulae. It ascends in this munner, 

 covering half of the ambulacral field to the summit of the calyx, 

 where it surrounds in a very acute angle two of the ovarian open- 

 ings and descends in a like manner on the following ambulacral 

 field (see Plate a. Fig. 10). These acute points, which almost 

 come in contact with each other at the summit, are the only cov- 

 ering of the central opening, which was never closed by additional 

 plates, as intimated by some authors* ; although specimens are 

 frequently found (and I have such in my collection) where it ap- 

 pears as if the summit were closed by additional plates, which, 

 on close examination, however, prove to be Bryozoa or ovulum- 

 like bodies. Again, it seems improbable to suppose that the cen- 

 tral opening was closed if we compare our fossils with Echino- 

 derins^ with which they have unquestionably most afiinity both 

 as regards the calcareous shell as well as the interior of the vis- 

 ceral cavity (except as regards the number and arrangement of 

 pieces). This can easily be comprehended if we divide the calyx 

 into two equal halves, i.e. an upper or dorsal and an under or 

 ventral one, of which the ventral one would be composed of the 

 pelvis and fork pieces, and the dorsal one of the deltoid pieces 



* Billin"S ;hkI Shumard. 



