112 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



or along the ambulacra! grooves, especially toward the mouth. In a few genera 

 similar plates are developed in the brachial perisome between the inner ends of 

 the brachials. 



The ambulacra of the arms and pinnules are often bordered by two rows of 

 small thin plates, the outer lying on the pinnules along the ventral edge of the 

 pinnulars, squarish or oblong, each usually with a notch at the distal proximal 

 corner for the reception of the sacculi, the inner, lying just within these and usually 

 in preserved specimens folded down so as completely to roof over the ambulacral 

 grooves, directed obliquely forward, rounded anteriorly, more or less pointed 

 posteriorly like a melon seed. The plates of the first or outer row are known as 

 Side plates, while those of the second or inner row are known as Covering plates 

 (see figs. 7, p. 63, and 55, p. 81). Covering plates occur alone in the Comasteridse, 

 but in the other families the two rows are either both present, both rudimentary, 

 or both entirely absent. 



These plates are similar in origin and significance to the concretions on the disk, 

 differing only in the greater regularity of size and shape. The two types are con- 

 nected by intermediate types bordering the ambulacra of the arms and of the 

 disk (see figs. 18, 19, p. 67). 



It is interesting to note a close connection between the development of the side 

 and covering plates and the development of concretions upon the disk, for when 

 side and covering plates are present the disk is always more or less heavily plated, 

 and when side and covering plates are rudimentary or absent the disk is, with 

 rare exceptions, almost or quite without plates or visible concretions. 



(8) Along either side of the free undivided arm is a row of slender and tapering 

 articulated processes, alternating in position, the Pinnules (see figs. 1, p. 60, 2, p. 61, 

 3, p. 62, 45, p. 79, and 78, p. 131). When the division series consist of four 

 ossicles the second always bears a pinnule on the outer side (see fig. 81, p. 134); 

 pinnules are never found on the ossicles immediately succeeding axillaries (see 

 following paragraph) nor on the hypozygals of syzygial pairs (see below). The 

 first pinnule is always developed on the outer side of the second ossicle of the 

 arm or division series which bears it. 



In the comasterid genera Capillaster and Nemaster curious exceptions to the 

 rule of pinnulation are found; the first and second division series are as usual, 

 but the third (IIIBr) and subsequent division series are of three ossicles (the two 

 outer joined by syzygy) of which the first bears a pinnule; on all arms springing 

 undivided from the second division series (IIBr), or beyond, the first brachial bears 

 a pinnule on the outer side. 



The first one to four or five pinnules on either side of the free undivided arm, 

 and all preceding pinnules, always differ from those succeeding in length and in 

 proportions ; they usually lack the ambulacral groove, being physiologically tactile 

 organs. In life they are bent over the disk instead of being laterally extended 

 like the others. These are known as Oral or Proximal pinnules (see figs. 1, p. 

 60, 6, p. 63, 83, p. 136, 85, p. 139, 104, p. 167, and 107, p. 173). They exhibit a 

 great amount of diversity in the different groups and hence furnish characters of 

 the greatest systematic value. In the Comasteridse the oral pinnules are provided 



