FOSSILS OF THE BURLINGTON GROUP. 387 



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 spines. 



W- have never seen any figures or specimens of the European typical 

 species of Amphoracrinns showing the arms, but there are in Mr. 

 WACHSMUTH'S collection several beautiful specimens, found at Bur- 

 lington, agreeing exactly in all other characters (not merely specific) 

 with the types of the genus, in which the arms are perfectly preserved. 

 In these we observe marked differences, both from Dorycrinus and Ac- 

 tinocrinus. For instance, in Amphoracrinus divergent (=Actinocrinus 

 divergent, Hall), the arms, although bifurcating so often as to form alto- 

 gether about Jifty -three or more divisions, are each, as well as each of 

 these divisions, composed of a double series of very short alternating 

 pieces all the iray doicn, even beloir all tlie 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 Saccocri- 

 nu* (an otherwise different type), but widely different from what we 

 see in Actinocrinus, in which the arms are always composed of a single 

 series of pieces beloir such bifurcations as take place. It is also even 

 more decidedly different from that of Dorycrinus, 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. (Actinocr inns spinobrachiatus, Hall), having 

 apparently all the other characters of Amphoracrinus, the arms do not 

 bifurcate after their origin on the axillary secondary radial pieces. It 

 seems, therefore, probable that there are some species of this group 

 with simple arms, and others that have them more or less frequently 

 bifurcating. The probability 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 bifurcations as well as above, the same as 

 in the species dirergen*. Whether we include such species as the spino- 

 brachiatus. however, with merely simple arm-*, composed like the others, 

 of a double series of alternating pieces throughout their entire length, 

 as a separate section of Anchor acrinus, or view them as forming a dis- 

 tinct group, they need not for a moment be confounded with Dorycrinus, 

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

 the vault, etc., but also in never having two arms springing directly 

 from each arm opening. 



In all the species of Amphoracrinns known to us, either foreign or 



