﻿wood] new and old middle devonic crino 69 



DOLATOCRINUS Lyon 

 1857. Lyon, Geol. Rep't Kentucky, in, p. 482. 



The number and arrangement of the anus has long been considered 

 of fundamental importance for the separation of species in the genus 

 Dolatocrinus, but the present study has. led the writer to regard this 

 feature as of less value for purposes of classification. Sonic of the 

 reasons which have led to this view are as follows: We find in- 

 dividuals of the genus which would be referred to the same species 

 except for differences in the number of the arms. The differences 

 in the arms vary in degree from a slightly unlike arrangement to wide 

 variation in both number and arrangement. For example, specimen 

 No. 672 in the collection of the Massachusetts Institute of Tech- 

 nology has the arm formula 1 2-1 . . . 2-2 . . . 2-1 . . . 2-2 . . . 2-2, while 

 that of Dolatocrinus venustus M. and G., which it otherwise resemblo. 

 is 1-2 . . . 2-2 . . . 1-2 . . . 2-2 . . . 2-2. Another specimen in the same 

 collection, identified as D. greenei, has the following arm formula : 

 2-2 . . . 2-2 . . . 2-1 . . . 2-2 . . . 2-2. The formula for the type is 

 2-1 ... 2-2 . . . 2-2 . . . 2-2 . . . 2-2, from which it will be seen that 

 the two specimens differ only in the grouping of the arms with refer- 

 ence to the anterior ray. 



Differences in the number of arms are illustrated by D. triadactylus 

 Barris and D. hammelli M. and G. The former has three arms to 

 the ray (1-2 . . . 2-1 . . . 1-2 . . . 2-1 . . . 1-2), and the latter one more 

 arm, thus, 1-2 . . . 2-1 . . . 2-1 . . . 2-1 . . . 2-2. A similar difference 

 exists between D. salcbrosus (see below) and a specimen which has 

 been referred to that species. 



So far as can be determined from the figures and descriptions, D. 

 bellulus M. and G. differs from D. nodosus M. and G., and D. 

 aspratilis M. and G. from D. argutus M. and G. only in the presence 

 of one additional arm. A greater difference is shown by D. sacculus 

 M. and G. and D. salcbrosus M. and G. The former has four arms 

 to each ray while the arm formula for the latter is 2-2 . . . 2-2 . . . 

 \-2 . . . 1-1 . . . 1-2 ; yet the two are closely related if not identical 

 forms. The great variation in the number of arms in this genus 

 is further illustrated by the occasional presence of a fifth arm in one 

 ray as in D. neglect us M. and G. The arms are arranged as follows : 

 1-2 . . . 2-1 . . . 2-3 . . . 2-1 . . . 1-2. It is believed that a compari- 

 son of the type specimens would furnish many illustrations of this 



1 The arm formulae used in this paper represent the number of arms begin- 

 ning with the anterior ray and passing from left to right around the tegmen. 

 The halves of a ray are separated by a hyphen, and adjacent rays by a row oi 

 dots. 



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