UINTACRINUS: ITS STRUCTUEE AND RELxVTIONS. 63 



}ieight of calyx and arms complete from the maximum adult size of 

 50 mm. down to 10 mm. 



Here, as elsewhere, the length of the brachials changes but little 

 with growth, but the enlargement takes place transversely, — the width 

 of the brachial increasing with a fairly regular progression, — so that the 

 ratio of width to length increases from \ in the youngest specimen to | 

 in the oldest. 



Similar measurements on a series of Forbesiocrinus will be found later 

 on, showing the same result (Table E). 



Dr. W. B. Carpenter, in his account of the Development of Antedoii 

 rosacea* says that in the young Aniedoii the form and proportions of the 

 arm segments vary widely in different parts of the arm; — "their length 

 being four or five times their diameter near its extremity ; less than twice 

 their diameter about its middle ; less than half their diameter near its 

 base ; this diversity almost entirely results from the progressive increase 

 in diameter, which shows itself in tlie segments as we pass from the 

 extremity toward the base, the absolute length of the segments being 

 nearly the same throughout." The same rule obtains in Uintacrinus, as is 

 shown by the measurements of brachials in different parts of several arms 

 already given. That is to say, the newer the segment the longer it is 

 in proportion to its diameter or width, and the relative as well as abso- 

 lute increase in diameter in parts of the arm of the same individual is 

 due to age. We would confidently expect to find the same rule applj-ing 

 as between corresponding brachials in j'oung and adult specimens, and 

 we find the fact stated accordingly by Dr. Carpenter on page 734 of the 

 same paper: "The average length of the basal segments of the arms at 

 this (early) stage of Pentacrinoid life is about .007 inch, and their diameter 

 about .003 inch ; whilst in the adult Antedon their length averages about 

 .03 inch, and their diameter about .07." 



I have applied this test to a number of specimens of Uintacrinus from 

 both Localities 1 and 2, vpith the results shown by the subjoined table 

 of measurements. This gives the width of the calyx at about the top 

 of the fii'st intersecundibrachs, and the length and width of the brachials — 

 or arm segments of W. B. Carpenter — at about the 10th secundibrach. 

 As tlie adjacent brachials are not always of uniform length, I have in 

 measuring the lengtli taken the average of several successive plates. All 



» Philos. Trans. 1866, p. 717- 



