552 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXII. 



of the basal cup is about equal to its width where it joins the radials, 

 and to twice its diameter at the dorsal pole. (See fig.) 



Primary radials closely united, the interradial sutures usually dis- 

 tinct, but in some cases nearly obsolete; basi-radial sutures usually 

 rather more distinct than the interbasal. The primary radials are 

 elongate, the median part of each one raised into a rounded ridge, 

 giving the caljx a rounded pentagonal aspect when viewed dorsall3\ 

 About two-thirds of the center of the superior edge of the primaiAp: 

 radials is occupied b}^ the first brachials: but the radials are not 

 especially incised for their reception 



PTILOCRINUS PINNATUS. a, crown and upper part op stem. 6, MIDDLE OF STEM. C, LOWER PART 

 OF STEM. i NAT. SIZE. SEE ALSO PLATE LIII FOLLOWING PAGE 556. 



The arms are five in number, and are unbranched. They consist of 

 about sixty joints, with pinnules on (in most cases the left of) the third 

 and all succeeding. The first brachial is usually a syzygy, as are the 

 fourth and sixth, with another after about ten joints, and two or three 

 at irregular intervals toward the tip of the arm. The longest pinnules 

 are between the twentieth and twenty-fifth, these having about thirty 

 joints. The pinnules decrease gradually in length toward the calyx, 

 the first having about twenty joints; distally the decrease in length is 

 ^•ather more rapid, the pinnules at the end of the arsis having about 

 ten joints. 



The longest stem is 355 mm. in length, and is composed of 360 joints, 

 very uniform in size, but becoming somewhat lower toward the calyx, 



