﻿68 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



Column circular with minute canal. The single specimen found 

 measures 10 mm. in height and 13 mm. in greatest diameter. 



Remarks. — In this specimen the presence of two plates in the 

 radial series of the anal interray is a remarkable feature, and were 

 that the only variation from a normal Megistocrinus it might be con- 

 sidered an abnormality, but the plates of the anal interray are more 

 numerous than in Megistocrinus, there being twenty-six plates in this 

 specimen as compared with seventeen in Megistocrinus tuberatus 

 n. sp., the species to which this is most nearly related, and the latter 

 is four times as large as Tylocrinus novus. It might also appear 

 that the wide posterior area is due to some accident which has 

 displaced the left posterior arms, and crowded out the adjacent 

 interbrachials and brachials, were it not that the elimination of calyx 

 plates begins only above the first interbrachial while the widening 

 of the anal interray begins in the radial series. Some difference in 

 the size or arrangement of the internal organs may have caused a 

 widening of the posterior area, necessitating the growth of more 

 plates to cover this portion of the body. The crowding out of lateral 

 plates would thus be a result and not a cause of the wide posterior 

 area. 



Of the two posterior plates in the radial series the left is orna- 

 mented like the radials with an elevated rim and central depression, 

 while the right bears a node like those of the higher calyx plates. 

 This fact seems to indicate that the right is the true anal plate, and 

 the left is a higher interbrachial which has moved downward to its 

 present position. 



Whatever the cause of the variation, the present species possesses, 

 in addition to the single extra plate in the radial series, a large num- 

 ber of well developed and regularly arranged posterior plates, whose 

 presence has affected in a marked degree other structures of the 

 calyx. Moreover, if two of the arms have been lost by crowding, as 

 the structure of the left antero-lateral ray indicates, this species was 

 derived from a form having four arms in each of the antero-lateral 

 rays and two in the others, the reverse of the arrangement common 

 in Megistocrinus, of two arms in the antero-lateral rays and four in 

 the other three rays. Such important structural differences from 

 Megistocrinus, its nearest ally, seem to entitle this form to rank as 

 the type of a distinct genus, of which Tylocrinus novus is the only 

 known species. 



Formation and locality.— Upper Traverse limestone: Partridge 

 I '.nut, near Alpena, Mich. 

 Cat. No. 35.150 U. S. X. M. 



