62 



UTNTACRINUS: ITS STRUCTURE AND RELATIONS. 



specimens gives the following result on the last point, in percentage 

 of total width of calyx : — 



Specimens 35 mm. and less . . . BB 16% 

 Specimens 50 mm. and more . . . BB 12% 



RR 36% 

 RR 27% 



But these, and the other differences to be noted, are for the most 

 part precisely the ones which result from differences in age. 



I have come to the conclusion that all these small specimens from 

 Locality No. 2 are the young of U. socialis. This opinion is based on the 

 following reasons : — 



In the first place, their calyx plates are extremely thin. In a large 

 proportion of the specimens from Locality No. 2 the plates of the plumpest 

 specimens are eroded on the convex portions, and in many of them alto- 

 gether. In several of the young ones from Locality No. 1, and of those 

 from Locality No. 2 as well, the plates of the interbrachial areas are so thin 

 that the black carbonaceous lining shines through them at the sutures. In 

 the second place, the arms have the undeniable characteristics of the 

 young. It is a fact evidenced by numerous observations that in Palaeozoic 

 species the brachials of young individuals are proportionally much more 

 elongate than those of the adult. This is well shown by Platycrinus 

 huntsvillce^ a species which is found in various stages of growth from the 

 very youngest, with uniserial, zigzag arms, to full grown ones, with arms 

 biserial down to the bifurcation. The following table of measurements 

 was taken from a series of eleven well-preserved specimens, ranging in 



TABLE C. 



Measurements of Platycrinus huntsvillce. 



Specimen. 



Height of 

 Crown. 



Height of 

 Calyx. 



Diameter of 

 Calyx. 



Brachials. 



Length. 



Width. 



1 



10 mm. 



2.50 



4.0 



.50 



.50 



2 



12 



3.00 



5.0 



.52 



.62 



3 



17 



5.00 



6.0 



.57 



.75 



4 



22 



6.00 



7.0 



.55 



1.00 



5 



25 



6.00 



8.0 



.55 



1.25 



6 



28 



7.00 



9.5 



.55 



1.25 



7 



31 



9.00 



12.0 



.55 



1.25 



8 



32 



9.00 



11.0 



.55 



1.50 



9 



34 



10.00 



13.0 



.55 



1.50 



10 



44 



15.00 



15.0 



.75 



1.75 



11 



50 



15.00 



17.0 



.62 



1.75 



