372 PROCEEDINGS OF THE ACADEMY OP [1881. 



1879. Zittel. Handb. der Palseont., i, p. 368 (in part). 



(Not Pacht, 1853. Verb. Kaiserl. Russ. Gesellscb., p. 262.) 

 Syn. Thysanocrinus Hall, 1852. Paleont. N.York, ii, p. 188. 



(Not Thysanocrinus {Bhodocr.) Billings. Geol. Surv. Canada for 

 1853 to 1856, p. 2o2.) 



The name Dimerocrinus was given by Phillips to two.species 

 from Dudlej", England, which are known as D. decadactylus and 

 D. icosidactylus, and which were figured without specific or 

 generic definition. The genus has been generally accepted, but 

 is since described as having only one ring of plates beneath the 

 radials, and this variously by different writers as composed of 

 either three or five plates. 



We have carefully examined the two species, and find that they 

 both have underbasals. Those of D. decadactylus are placed 

 within a rather deep concavity, formed between the basals and 

 hidden by the column, exactly as some species described and 

 figured by Hall under his genus Thysanocrinus, which we take to 

 be a sj'nonym of Dimerocrinus. Phillips' D. icosidactylus is 

 geuericall}' distinct, and has been referred by us to Eucrinus. 

 Ver}' closely allied is Patelliocrinus Angelin, which Zittel unites 

 with Dimerocrinus; the two genera resemble each other most 

 remarkably, but the former can readily be separated by its three 

 basals and the absence of underbasals. 



Generic Diagnosis. — Body small, calyx short, subglobose or 

 conical ; plates not numerous, surface less ornamented than in the 

 preceding genera, smooth or indistinctly granulose ; symmetry 

 bilateral. 



Underbasals five, small, arranged within a concavity, which is 

 nearly or entirely filled by the column. Basals five, four of them 

 equal, angular above ;i the fifth truncate and supporting the first 

 anal plate. Primar}^ radials 3X5; the first almost as large as 

 the second and third together and considerably wider, the two 

 lower sides making an angle, which rests deeply between the 

 basals ; lateral sides short, the first interradial is almost touching 

 the angle of the basal plates ; second radials more or less quad- 

 rangular, wider than high ; the third radials giving off two or 

 three secondary i-adials, of the form of arm plates, of which each 

 one supports an arm. . Pinnules strongs less closely arranged than 

 usuallj" in this family, their joints rounded, two or tlai*ee; times as 

 long; as wide. 



