PART 5 A MONOGRAPH OF THE EXISTING CRIXOIDS 649 



deeply excavated by a posterior process from the axillary, sometimes so much so that 

 when viewed at right angles to the dorsoventral axis the IBri appears bisected. The 

 IBr 2 (axillaries) are usually rhombic and about as long as broad, frequently some- 

 what broader than long, in one case as much as half again as broad as long, but rarely 

 longer than broad. The distal sides are more or less strongly concave, and the prox- 

 imal sides are usually somewhat less concave. The distal angle is usually sharp, 

 but often truncated, the truncation sometimes with a notch from which a shallow 

 groove runs backward over the dorsal surface. The lateral angles may extend 

 considerably beyond the laterodistal angles of the IBr^ 



The IBr series usually have a high and strongly arched dorsal surface and rounded 

 sides; when the sides of the IBri converge distally the lateral angles of the axillaries 

 project for some distance beyond their laterodistal angles. The IBr series may be 

 widely separated, or the lateral angles of adjacent axillaries may be in contact, with 

 large rhombic open spaces below them. Sometimes the IBr series are much broadened 

 and less convex dorsally and the edges of the component ossicles are everywhere in 

 contact with those of their neighbors against which they may even be broadly and 

 sharply flattened. 



The proximal and distal borders of the elements of the IBr series are usually 

 smooth, but there may be a greater or lesser development of spines, especially at and 

 near the lateral angles of the axillaries. In only a single species (Tonrometra brevipes) 

 is the dorsal surface studded with small spinules. 



The arms are invariably 10 in number; no case of an individual with more than 

 10 arms has ever come to light, nor is there any record of an individual with loss than 

 10 arms. 



In length the arms range up to 130 mm., being most commonly between 20 and 

 60 mm. In only 3 or 4 species does the arm length exceed 90 mm. 



The first brachials are always very short, with the outer edge from half again 

 to 4 or even 5 times (usually about twice) as long as the inner, and the distal border 

 more or less deeply excavated. The inner borders are usually entirely free, but they 

 may be basally in contact, or even united for half their length. 



The second brachials are irregularly 4-sided or triangular and much larger than 

 the first, usually about as long as broad, rarely longer than broad. 



The first syzygial pair is usually somewhat longer interiorly than exteriorly, and 

 in extreme cases the interior length may be as much as twice the exterior. The prox- 

 imal edge of the hypozygal is always more oblique than the distal edge of the epizy^al. 

 Generally this pair is nearly oblong, somewhat broader than long, but it may be as 

 long as broad, or even almost twice as long as broad, as in Bathymetra. 



The 4 to 6 brachials following the first syzygial pair are oblong to moiv or less 

 wedge-shaped, broader than long to nearly as long as broad, and the following become 

 very obliquely wedge-shaped or triangular, about as long as broad or slightly longer 

 than broad, and distally gradually less obliquely wedge-shaped and elongate, finally 

 centrally constricted with swollen ends. 



The ends of the oblong or wedge-shaped earlier bra'chials may be smooth or bor- 

 dered with fine spines; they are sometimes abruptly everted, standing out at right 

 angles to the dorsal surface as high spinous ridges, or they may be thickened and 

 smooth. The distal edges of the outer brachials are usually somewhat produced and 

 finely spinous, more rarely coarsely spinous. 



