30  RHIZOCRINUS RAWSONII. 



progress of the earth's growth we must look t<j such a dis[)lacement of the conditions 

 favorable to the maintenance of certain lower types, as may recall most fully the adapta- 

 tions of former ages ; and it was in this sense I alluded, in my first letter to the Super- 

 intendent of the United States Coast Survey, to the probability of our finding in deeper 

 waters representatives of earlier geological types. If my explanation is correct, my anti- 

 cipation is also fully sustained. But do the deep waters of the present condition of our 

 globe really approximate the conditions for the development of animal life of the shoaler 

 seas of past geological ages ? I think they do so ; at least they come as near to it as 

 anything can in the present order of things upon eartli ; since depth in the ocean alone 

 can place animals under conditions similar to those produced by the high pressure which 

 the heavy atmosphere of earlier periods afibrded." 



Concerning the geographical distribution of this species we know, of course, 

 nothing yet. All the specimens from Florida in the Museum of Comparative 

 Zoology belong to R lofotensis, which, from what Ave know from various 

 explorations, inhabits a large portion of the North Atlantic. It is worthy of 

 remark that all the Florida specimens dredged by me are regularly five- 

 armed. It is well known that a large proportion of those obtained on the 

 coast of Norway by Sars were either four or six armed. 



The fossil from Guadaloupe, on which D'Orbigny has founded his species 

 of Bourgueticrinus Hotessieri, consists of a fragment of stem composed of six 

 cylindrical joints. If they originally belonged to a Rhizocrinus, they could be 

 only the joints immediately below the cup, which are the ones most recently 

 formed. But in our Rhizocrinus a series of six joints of that shape are not 

 formed in that situation; they become rapidly elongated and swelled out in 

 the middle as we follow the stem downwards. Another difference is that 

 the central hole is never as large as in D'Orbigny's figure. 



Among the older fossil Crinoids, the nearest approach to Rhizocrinus 

 can be found in Belemnocrimis, White, from the upper Burlington Limestone, 

 Iowa. (Proc. Boston Nat. Hist. Soc, Vol. IX. p. 13.) I have had the oppor- 

 tunity to examine several specimens of B. typus and one of B. Whitei, in the 

 rich collection made by Mr. Wachsmuth and lately acquired by the Museum 

 of Comparative Zoology. The former species has a calicle resembling that 

 of Rhizocrinus very closely ; the second differs only by being more obese. 

 I am satisfied that the five short basal pieces figured by Mr. White have no 

 existence, and am inclined to think he has mistaken for them the upper joint 

 of the stern which is somewhat indented in its upper edge to fit the base of 

 the calicle. Moreover, that articulation can be seen plainly in one or two 



