1<S90.] NATURAL SCIENCES OF PHILADELPHIA. 361 



paroiitly i-est upon tlie lateral iiiar<i'ins of the five large plates. There 

 are no orals, the median portions of the disk being occupied by large 

 covering plates. The anus is closed by a pyramid of six or more 

 pieces, surrounded by numerous irregular smaller plates. 



The ventral pavement of the Anomalocrinidae, which we have 

 observed in a fine specimen from the Museum of Comparative Zool- 

 ogy at Cambridge,^ is very dirterent from that of the Cyathocrinidae. 

 It consists of rather large, moderately thick, irregular pieces, five or 

 six deep, which decrease in size toward the center and cover the 

 peristome, there being no orals or consolidating plates. The outer 

 plates rest against the large incurved limbs of the radials, which 

 form a wide and deep sinus at the upper face of the calyx for the 

 recej:)tiou of the ambulacra, which come out beneath the irregular 

 calyx plates above, as beautifully shown in the specimen. Not 

 only are the ambulacra! plates on a lower level than the disk plates, 

 but they are also of a difl^erent shade of color, which makes the case 

 more instructive. 



In the Poteriocrinidae, the ventral pavement has never been found 

 in perfect preservation. We have dissected a number of finely pre- 

 served specimens, but only in one instance found small fragments of 

 the tegmen in situ, which indicate that it consisted of very small 

 delicate pieces. The plates of the ambvdacra must have been also 

 extren)ely fragile, for with the exception of the one instance above 

 mentioned, we never found them preserved either upon the disk or 

 upon the arms, owing probably to the absence of any consolidating 

 ap])aratus. The form and size of the ventral sac is extremely vari- 

 able in this group. In some genera it extends beyond the tips of the 

 arms, in others consists merely of a short cone, in some of them it 

 is tubular, in others club-shaped, in some balloon-shaped or coiled 

 uj), the sides of the whorls connected or free ; but in all eases, so far 

 as known, the plates are arranged in vertical rows which sometimes 

 diverge at intervals, and in all of them the edges of the plates^ — not 

 their substance — are perforated by pores or clefts. The anus, 

 wherever it has been observed, is located at the anterior side of the 

 sac, and generally well down toward the bottom. 



1 Prof. Alex. Agassiz has given us the unrestricted use of the magnificent Cri- 

 noid collections of the Museum of Comparative Zoology at Cambridge, containing 

 several of the finest original and typical collections ever made from different forma- 

 tions, both in this country and Europe. Only those who are acquainted with the 

 extent and variety of the material thus brought together can appreciate the obliga- 

 tion under which his liberaliiy has laid us. 



