478 PROCEEDINGS OF THE NATIONAL MUSEUM. tol. 46. 



tains a fairly uniform diameter. At about 5 mm. from the theca the 

 column has a diameter of 0.3 mm. In the sharply narroAving por- 

 tion of the stem the ossicles are comparatively low and apparently 

 not differentiated into nodals and internodals. In the next milli- 

 meter there are alternating wdde and narrow ossicles of about the 

 length of those noted above. Distad from this area the columnals 

 are considerably longer and of approximately equal size. From these 

 facts it may be inferred that in adult specimens at any rate increase 

 in the length of the stem by the intercalation of new columnals took 

 place chiefly in that portion of the column lying immediately distad 

 to the proximal group of tapering ossicles. The column attained a 

 length of perhaps five or six times that of the crown. The extreme 

 distal portion of the stem has not been observed. 



The geological horizon of Eomocrinus parvus is at the top of the 

 lower third (lower 17 feet) of the Kochester shale (Niagaran), accord- 

 ing to Eingueberg (1888, p. 269). It has only been recorded from 

 Lockport, New York, where it is found associated with characteristic 

 Niagaran foss'ls. 



It is difficult exactly to establish the relationships of Homocrinus . 

 On the whole, the affinities of the genus seem to be closest to the 

 Heterocrinidae, and the genus might well be placed here were it not 

 for its simple, nonbifurcating arms. The simplest type of arm among 

 the Heterocrinidae is isotomous. It is obviously impossible to derive 

 Homocrinus from any known form referred to the family on this 

 account. Ectenocrinus, which precedes Homocrinus by a considera- 

 ble period of time, is the form to which Homocrinus is most closely 

 comparable structurally. Indeed the arrangement of cup plates in 

 the two genera is essentially identical. The later form, however, has 

 the more simple arms. We may postulate a common ancestor for 

 Ectenocrinus and Homocrinus. Such a form would probably partake 

 more nearly of the nature of Homocrinus than any other known genus. 

 Were the geological positions of the two genera reversed one might 

 well consider Homocrinus not far out of the ancestral line which 

 evolved Ectenocrinus. We have illustrated here a case of the primitive 

 ancestral type surviving with perhaps few marked modifications long 

 after the extinction of more complex derivatives of the parent stock. 



Among the genera of contemporaneous and subsequent geological 

 occurrence Homocrimis occupies a somewhat anomalous position. 

 The genus has a similar arm structure to that of the Pisocrinidae and 

 Haplocrinidae. The cup has the essential arrangement of plates of 

 Haplocrinus as well, with the exception of x reaching down into the 

 cup, as in the Heterocrinidae. This indicates the presence of an anal 

 tube and a type of tegmen quite at variance with that of Haplocrinus. 

 Under the circumstances it has seemed best to establish a new family 

 Homocrinidse for the reception of the genus. This family may be 

 defined as follows: 



