272 SMITHSONIAN INSTITUTION 



mentioned, and to avoid tedious repetition this is not further stated in the description ; thus 

 if the data given is for a radial, or a non-axillary brachial, it means at the upper margin of 

 that plate ; if for an axillary, the height is taken at the apex. Also in this genus and others 

 of this family for purpose of better comparison I measure the. spread of the calyx from the 

 top of the radial, instead of from the base or column facet as is usually done in those of other 

 families. 



The size of the crown varies considerably in the different species. /. corbis and I. laevis 

 attain a large size, though small specimens are found, especially of the latter species, showing 

 marked juvenile characters. I. subangularis is also a fair-sized species. /. piriformis and 

 /. gotlaiidicus are generally small. 



The figures given show the average of the available specimens, and my experience with 

 this genus is that in relative proportions individual specimens do not depart very widely 

 from the mean. It is of little use to give elaborate measurements or proportions of separate 

 plates, as they vary too much in the same specimen. In a general way we find that in a 

 rapidly spreading species, like 7. corbis, the brachials are proportionally shorter and wider 

 than in an elongate, less expanded form like /. subangularis. 



The ten species may be arranged as follows : 



The Species of Ichthyocrinus 



Silurian Species 



I. Surface ornamented. Calyx without radial ridges. 

 Crown pyriform. 



BB well-developed. IIBr usually 4 I. piriformis. 



Crown ovoid. 



BB small, covered by column. IIBr 3 . .1. gotlaiidicus. 



II. Surface smooth. 



Crown pyriform. 



BB small, visible as triangles in lateral view. 



Calyx rounded, with low radial ridges. IIBr 4 I. laevis. 



Crown turbinate or conical. IIBr 4. 

 BB large, visible as pentagons. 



Calyx angular, with strong radial ridges. 



Base contracted to a truncate neck /. subangularis. 



Base rounded or curving evenly with calyx wall /. intermedins. 



BB small, visible as triangles. 



Calyx narrowly elongate without radial ridges /. conoideus. 



Crown ovoid. 



BB small, covered by the column. 



Calyx angular, with radial ridges. IIBr 3 /. phillipsianus. 



Calyx rounded, with obscure radial ridges. IIBr 4 I. corbis. 



Devonian Species 



Surface smooth. Calyx without radial ridges. 

 Crown pyriform. 



BB small, covered by the column. IIBr 3 7". devonicus. 



Crown ovoid. 



BB unknown. IIBr 3 1, bohemicus. 



