138 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



carries a large and thick club spine, which broadens toward its ex- 

 tremity into a head bristling with rather closely crowded spinules, 

 the cylindrical stem being simply rugose (pi. 94, fig. 7). The length 

 of these club spines is equal to four times their thickness. In my 

 specimen very many of the plates have lost the club spines, but it is 

 easy to recognize the mark representing their insertion. These club 

 spines also occur on the plates which separate the two radial shields 

 of each pair, and some even pass onto the first upper arm plate ; they 

 become a little smaller and more slender toward the periphery of the 

 disk. The radial shields are relatively rather small, triangular, 

 almost as broad as long, and divergent; they are close together, 

 though not in contact in their distal portion; these shields are less 

 developed than in the majority of the species of the genus Ophio- 

 plinthaoa. 



The ventral surface of the disk is covered in the interradial spaces 

 by imbricated plates, the majority of which bear a spine which is a 

 little narrower than those on the dorsal surface, and which are also 

 found on the plates bordering the mouth shields. The genital slits 

 are rather narrow. 



The mouth shields have a lance-head form, with a very sharp and 

 produced proximal angle bordered by two very concave sides; the 

 lateral angles are very sharp and produced, the two distal sides are 

 shorter than the two others and they pass into each other over a 

 rounded angle. The adoral plates are not very large ; they are much 

 broadened outwardly and gradually narrow toward the median in- 

 terradial line along which they are in contact by their internal an- 

 gles. The oral plates are small; the lateral mouth papillae, four in 

 number, are rather stout and thick; the two external papillae are 

 approximately cylindrical, and slightly swollen at their strongly 

 rounded tip; the two others are more conical in shape, with the 

 point blunted. The unpaired terminal papilla is a little more elon- 

 gated than those on either side. All these papillae have their surface 

 very rugose, or even provided with extremely fine and closely set spi- 

 nules. An additional very small papilla is inserted at the external 

 angle of the first under arm plate. Between this and the external 

 mouth papilla there is a small group of granules identical with 

 that which H. L. Clark has described in 0. bytMaspis, and which I 

 have found myself in O. globata, as I have just said, as well as in 

 O. pulchra. This little group of granules is only slightly evident in 

 this individual; it is better indicated in the specimen from station 

 5280. 



The upper arm plates, which are of medium size, are triangular, a 

 little longer than broad, with an acute proximal angle, slightly con- 

 vex sides, and a gently rounded distal border. Beyond the second 

 they are separated by a very narrow interval. 



