PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 459 



median line. The plane of their mid-dorsal line makes a broad angle, sometimes 

 approaching 90°, with the dorsoventral axis, imparting a very characteristic appear- 

 ance. The distal angles of the radials arc separated by a slight notch, and their borders 

 may be smooth, or more or less everted and tubercular. 



The IBr, are from two and a half to four (usually three or four) times as broad as 

 long, oblong, or ^vith more or less converging sides, with the borders unmodified, or 

 with the lateral edges coarsely spinous or tubercular, and the proximal and distal 

 edges sometimes everted and spinous or tubercular. The distal border is usually more 

 or less depressed in the middle by a rounded posterior process from the axillary. 



The IBrj (axillaries) are always broad, from about as broad as long to nearly 

 twice as broad as long. Their lateral angles usually extend somewhat beyond the 

 anterolateral angles of the IBri. Their edges may be smooth, but there are commonly 

 ventrolateral processes or tubercles beneath the lateral angles, and the proximal and 

 distal borders may be everted and spinous or tubercular. The middle of the proximal 

 border is always extended downward more or less in the form of a rounded process 

 incising the distal border of the IBr,, and this proximal process, with the adjacent 

 portion of the IBrj, commonly rises into a marked synarthrial tubercle, which may be 

 (especially in Perometra) greatly exaggerated. 



Two quite different kinds of IBr series occur in this subfamily. In Perometra the 

 elements of the IBr series are quite smooth all around the edges and are in close lateral 

 apposition, mth their sides sharply flattened against those of their neighbors in the 

 manner characteristic of the species of Thalassometridae (see fig. 19, p. 463). In 

 the other genera the IBr series are narrower, and the lateral angles of the axillaries 

 extend more or less, though never very greatly, beyond the anterolateral angles of the 

 IBrj. In these genera the ventrolateral borders of the elements of the IBr series usually 

 bear flangehke processes or tubercles which may meet those of their neighbors. 



There is never any ornamentation on the dorsal surface of the IBr series. 



When IIBr series are present (only in Perometra afra), they are always of 2 ossicles. 



The first 2 brachials as usual reduplicate the essential features of the elements of 

 the IBr series; the second brachial is always larger than the first and irregularly quad- 

 rate in form. 



The division series and arm bases lie in planes making a relatively small angle 

 with the dorsoventral a.xis, so that the proximal portion of the animals appears more 

 or less compressed. 



The arms vary in length in adult individuals from 25 to 120 mm., and are usually 

 between 35 and 70 mm. The average length for all the species is 53 mm., and in only 

 one do they exceed 90 mm. 



The general structure of the arms varies rather more than in most of the sub- 

 families of Antedonidae. The first 2 brachials may be relatively large with the second 

 of approximatelj' the same shape as, and but little larger than, the fii-st (Perometra), 

 or the fii-st may be small and the second larger, sometimes considerablj' larger, as is 

 usual in the Antedonidae. 



The first brachial is usually twice, rarely as much as 3 times, as long exteriorly as 

 interiorly; it may be almost wedge-shaped, or tlie distal border may form an obtuse 

 angle in such a waj' that the inner half is nearlj' or quite parallel with the pro.ximal 

 border while the outer forms an acute angle with that border. 



