348 PROCEEDINGS OF THE ACADEMY OF 



group. Some of the species are known to have one of the vault pieces over 

 each ray more or less protuberant, and it may be the case that species existed 

 in which those were developed into spines, somewhat as in Dorycrimis, though 

 we are not aware that any such have yet been found. Near the middle of the 

 vault there is also a large tumid piece, sometimes developed into a short spine, 

 while around the anterior side of this, four or five similar pieces are semi- 

 circularly arranged, which also often become well developed sjiines. 

 We have never seen any figures or specimens of the European typical species 

 of Amphoracrimis showing the arms, but there are in Mr. Wachsmuth's collec- 

 tion several beautiful specimens, found at Burlington, agreeing exactly in all 

 other characters (not merely specific) with the types of the geniis, in which the 

 arms are perfectly preserved. In these we observe marked differences, both 

 from Dorycrinus and Actinocrinus. For instance, in Amphoracrmus divergens 

 (= Actinocrinus divergens^ Hall), the arms, although bifurcating so often as to form 

 altogether about fifty-three or more divisions, they are each, as well as each of these 

 divisions, composed of a double series of very short alternating pieces all the 

 way down, even below all the bifurcations to their very bases, with the exception 

 of one to two or three simple brachial pieces, at the origin of each main arm 

 on the last primary or secondary radials. 



The structure of the arms, it will be seen, is the same as in Saccocrinus (an 

 otherwise different type), but widely different from what we see \n Actinocrinus, 

 in which the arms are always composed of a single series of pieces beloio such 

 bifurcations as take place. It is also even more decidedly different from that 

 of Dorycrimis, in which the arms not only never bifurcate in any of the species in 

 which they are known, but are also arranged so that two of them spring 

 directly from each arm-opening without imparting to the brachial piece on 

 which they rest the usual form of an axillary .piece. 



In one species, however, [Actinocrinus spinobrachiatvs, Hall), having appa- 

 rently all the other characters of Amphoracrimis, the arms do not bifurcate 

 after their origin on the axillary secondary radial pieces. It seems, there- 

 fore, probable that there are some species of this group with simple arms, 

 and others that have them more or less frequently bifurcating. The proba- 

 bility is, however, that in all cases when they do bifurcate, that they are 

 equally composed of a double series of small alternating pieces below the bi- 

 furcations as well as above, the same as in the species divergens. Whether we 

 include such species as the spinobrachatus, however, with merely simple arms, 

 composed like the others of a double series of alternating pieces throughout 

 their entire length, as a separate section of Amphoracrinus, or view them as 

 forming a distinct group, they need not for a moment be confounded with 

 Dorycrimis, from which they not only differ in form, surface markings, opening 

 of the vault, &c., but also in never having two arms springing directly from each 

 arm-opening. 



In all the species of Amphoracrinus known to us, either foreign or Ameri- 

 can, it is also worthy of note that the surface of the body plates is never orna- 

 mented with proper radiating costne, such as we often see in Actinocrimis and 

 other allied types, nor yet smooth, or properly tuberculiform, as in Dorycrinus, 

 but always presents a peculiar vermicular style of sculpturing or corrugation 

 difficult to describe, but very characteristic and easily recognized again after 

 being once observed. 



To this group Mr. Wachsmuth has, in MS., correctly referred the following 

 American species, viz : Amphoracrinus divergens, A. planobasalis, A. spino- 

 brachiatus ? and -4. intiatus, described by Prof. Hall, under Actinocrinus; also 

 Actinocrinus quadrispinus, White ; all from the lower division of the Burling- 

 ton beds. 



Amphoracrinus divergens, Hall, (sp.) 



Actinocrimis divergens, Hall, Supp. Iowa Report, p. 36, 1860. 



This species was apparently described by Prof. Hall from imperfect speci- 

 mens, showing only a few of the lower bifurcations of the arms, since he 



[Dec. 



