30 RHIZOCRINUS RAWSONII. 



- of the earth's growth we must look to such a displacement 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 tli" United States Coast Survey, to the probability of our finding in deeper 

 waters representatives of earlier geological types. It' 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 pasl 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 earth; 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 afforded." 



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

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

 Zoology belong to R. lofotensis, which, from what we 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 live- 

 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 BourgueticrinilS Hotessieri. consists of a fragment of stem composed of six 

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

 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 thai situation; they become rapidly elongated and swelled out in 

 the middle a- 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 Belemnocrin,U8, White, from the upper Burlington Limestone. 

 Iowa. (Proc. Boston Nat Hist. Soc, Vol. IX. p. 1."..) I have bad the oppor- 

 tunity to examine several specimens of B. typus and one of 1?. Whitei. in the 

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

 of Comparative Zoology. The former Bpecies lias a calicle resembling that 

 of Rhizocrinus very closely; tli" Becond differs only by being more obese. 

 I am satisfied that the live >hmt basal pieces figured by Mr. White have no 



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



of the stem which is somewhat indented in its upper edge to lit the base of 



the calicle. Moreover, that articulation can be seen plainly in one 01 two 



