2 Bulletin 23 28 



tinued up along the fusion of the rays throughout their length 

 and extending also into the two lateral branches. 



Remarks on Trichotocrinus 



The characters as given above show the intimate relation of 

 the form with Melocrinus. The structure of the arms is however 

 very different and as in some other members of the family, is a 

 measure of the amount of evolution through which they have 

 passed. Because only a single specimen is known and that 

 with only one arm preserved, it has been considered advisable to 

 give it only sub-generic rank. The family Melocrinidce does 

 not survive the Devonian, being represented in the Sub-Carbon- 

 iferous by the closely related family Adinocrinidce. For this 

 reason such variation from the typical Melocriuid arm is apt to 

 be looked upon as aberrant or regressive in nature. However, a 

 more ready explanation is found in considering it a natural re- 

 sult of evolution, which may be thought of as commencing with 

 Mariacrimis and passing through Melocrinus, two genera, which 

 differ from each other only in their arm characters. The pres- 

 ence or absence of interdistichal plates appears also to be a 

 measure of evolution, becoming eliminated as the arms become 

 more ridigly biserial. 



In Mariacrimis, the arms are typically uniserial and bear arm- 

 lets, with pinnules only on one side. The next step is the 

 fusion of its uniserial arms in the production of biserial ones, 

 which commences proximally, finally extending throughout the 

 entire length of the arms. Species occur in which this fusion 

 has been but partially completed.* The complete fusion of the 

 arm give rise to the genus Melocrinus (see Plate 6, figs. 4, 5, 6.) 

 with armlets arranged on opposite sides and which are usually 

 biserial. t The last step in this evolution would necessarily be 



* Mariacrimis beecheri Talbot. Revision of the New York Helderber- 

 g'ian Crinoids. Am. Jour. Sci. 4th series, vol. 20, p. 25, 26. proximal 

 portion biserial only, uniserial portion longest, hence referred to Maria- 

 crinus. 



t Melocrinus pachydactylus Con. Talbot, see above' p. 27, states that this 

 species has uniserial arms and not biserial ones as previously described and 

 figured. 



