100 University of Texas Bulletin 



TYPE INDIVIDUAL : A well preserved individual showing both sides 

 of the disk in great detail and having two complete and three fragmentary 

 rays. Museum of Texas Christian University, Fort Worth, Texas. 



DECRIPTION : Marginal plates distinct, subrectangular on edge 

 view, rounded on the face, bearing fine tubercles. Inter-marginalia pres- 

 ent in the interarcs and the proximal half of the rays, decreasing in size 

 outwards from the disk. Disk relatively small, arcs sharply rounded, rays 

 long and slender; oral surface flat with numerous small plates; aboral 

 surface with a central raised pentagonal system of plates and a row of 

 radiating plates on the midline of each ray. 



ABORAL SIDE : The rays are slender and the interarcs sharply 

 curved. Each interarc contains from tip to tip of the rays 48 supero- 

 marginal plates (17+7 in each half arc) . Of these all except the terminal 

 seven of each ray are elongate in the direction of the ray, sharply rounded 

 on top and separated from those of the opposite side of the ray by radial 

 plates. The terminal seven plates of each margin of the ray are apposed 

 along the mid-line; the plates are transversely elongate and join by alter- 

 nate facetting, along a zigzag line; the terminal plate is unpaired and 

 is common to the two rows. The seven terminal plates are swollen and 

 smooth and present a very different appearance from the others. In the 

 type they are smooth and whitish, lacking the iron stain of the rest of 

 the animal. A single row of radials extends from the inner pair of these 

 terminal plates to a polygonal elevation in the center of the disk, the plates 

 increasing in size towards the disk. Between this row of plates and the 

 supero-marginal plates there are two parallel rows of plates, an adradial 

 row of large plates fitting into the spaces between the radialia, and a 

 para-marginal row of small cuboidal plates lying against the marginal 

 plates, there being about three of these plates to one marginal plate. 



The interradial areas on the disk are triangular and are covered with 

 three or four rows of small plates lying close to the marginals; at the 

 apex of each area is a perforated, elevated polygonal plate, the genital 

 plate with its gonopore. The radial ridges running on the mid-line of 

 the ray near the disk are composed of three irregular rows of large rounded 

 ossicles and numerous smaller scattered plates, which were apparently 

 imbedded in a membrane. The central polygonal ring-like elevation at 

 their union bears scattered large and small ossicles. The anus is sub- 

 central and large ; the madreporite is excentric, raised, hexagonal in shape, 

 depressed in the center, and has coarse imbricated V-shaped ridges, and 

 straight, simple or branching ridges, radiating from the center. 



ORAL SIDE : The oral side is strikingly flattened, being only slightly 

 compressed along the ambulacral grooves. Its prevailingly smooth ap- 



