90 Mr. W. K. Fisher on two 



are intermediate in many respects between Lud wig's two 

 families. They resemble Luidiaster (Cheirasteridse) in the 

 structure of the abactinal spinopaxillffi, heavily armed mar- 

 ginals, and adambulacral plates, but possess also the 

 diagnostic feature of the other family — the odd interradial 

 marginal plates. For this reason I am inclined to maintain 

 tiie family Benthopectinidai in the extended sense, and 

 without subfamilies. 



Ludwig recognizes two genera with odd interradial mar- 

 ginal plates : Pararchaster, having the abactinal plates with 

 several spiuelets, and Benthopecten, with only one spine or 

 spinelet to a plate, rarely two or three. No other characters 

 are designated. The two groups are extremely close, and 

 often difficulty is encountered in deciding to which genus a 

 given form belongs. This is true of the variable B. acantho- 

 notus, Fisher, which is Benthopecten on the rays, and 

 Pararchaster over most of the disk. My B. claviger would 

 fall in Pararchaster and B. mutahilis probably in Bentho- 

 pecten, although many plates have several spineleta (1 to 7). 

 Unfortunately the type of Pararchaster is not so well 

 differentiated from Benthopecten as some other species of the 

 group — e. g., P. spinosissiinus. I have used the name 

 Benthojyecten in the subjoined synopsis in the extended sense, 

 equivalent to Sladen's (not Ludwig's) Pararchester. This 

 ke}' is intended to complement the second portion of Ludwig^s 

 synopsis (" Notomyota," p. 442). 



An unpaired (odd) marginal plate present in 



some or all of the interradii. Dorsal 



muscle-bands not attached by a tendon to 



a proximal ambulacral ossicle. 

 a. Abactinal platesof papular areas low, tabulate, 



and strongly stellate, the larger bearing a 



conspicuous central spine surrounded by a 



circle of more or less elongate spinelets 



(fig. 5, p. 92) ; superomarginals with 2 or 3, 



inferomarginals with 2 to 5 long bristling 



spines (fig. 3) ; odd interradial marginal not 



especially prominent ; usually absent in 



Bome of the interradii, never in all, and 



sometimes present in only one. 

 b. Papulae extending one-third to three-fifths 

 length of ray ; mouth-plates with nume- 

 rous (9 to 16) marginal spines disposed 

 in three independent series on each pair 

 of mouth-plates; the median teeth the 

 longest, and less numerous than either 

 farrow series (fig. 1) ; dorsal muscle- 

 bands not very strong or prominent, 

 often weak , Nearchaster, gen. nor. 



fi 



