388 



PROCEEDINGS OF THE ACADEMY OF 



[1890. 



In Taxocrinus and Onychocrinns, the plate X is followed by a 

 vertical row of additional pieces forming a lateral tube ; in Calpio- 

 crinus by one or two large plates longitudinally arranged, resting 

 between the radials ; in Forbesiocrinns by a great number of large 

 interbrachial plates ; in Mespilocrinus by one plate only. The anal 

 area of Sagenocrinus is similar to that of Forbesioerinus, that of Le- 

 canocrinus and Pycnosaccus like that of Mespilocrinus, the pres- 

 ence of the radianal excepted ; and Gnorimocrinus in its anal struc- 

 ture resembles closely Taxocrinus, but the tube of the former rests 

 obliquely, and not vertically, upon the plate X, and partly upon the 

 radianal. 



There has been considerable doubt as to the row of plates in the 

 anal interradius of Omjchocrimis and Taxocrinus, whether they are in- 

 corporated into the calyx, as the anal plates of Reteocrinus, or form a 

 free tube. We can now state from actual observation that the latter 

 is the case, at least in some species. At the special request of Dr. 

 Carpenter, we dissected some fine specimens, and in Onychocrinus 

 exculptus, 0. ulrichi, and in a large species of Taxocrinus, succeeded 

 in laying bare the tube at all sides. The posterior side consists 

 of a vertical series of eight or nine large subquadrangular plates, 

 loosely connected, and sometimes separated by very minute pieces. 

 The anterior side is composed of a large number of quite small per- 

 isomic plates, forming a sort of pouch, wider at the proximal end. 

 In all of those species, the tube at its anterior side leans considerably 

 to the right, and we suggest from this structure that both genera 

 were derived from the asymmetrical Gnorimocrinus, which occurs in 

 the Upper Silurian contemporaneously. The tube at its distal end 



