120 Rev. A. M. Norman on the Genera and 



placed by Diiben and Koren, and placed it in Archaster on the 

 authority of Professor Sars. I am unable myself to vouch for 

 the correctness of this transfer, as I have been unwilling to 

 injure the only British specimen in order to ascertain the pre- 

 sence of those organs (the anal aperture and pedicellarise) which 

 separate the genus Archaster from Astropecten. 



Oenus XIII. Palmipes, Linck. 



[Palmipes, Linck. 1733, and Agassiz, 1837; Asteriscus, Miill. & Trosch. 



(partly), 1840.] 



Body pentagonal, extremely thin and flat ; sides greatly pro- 

 duced beyond the central cavity in the form of a thin lamella. 

 Surface furnished with fascicles of spines (not paxillse). These 

 fascicles are arranged, especially on the under surface, in radiating 

 lines. Each adambulacral plate bears about five spines, the 

 central of which is the longest. The body and rays have an 

 acute edge unprovided with marginal plates or spines. Suckers 

 biserial. Anus subcentral. No pedicellarise. Madreporiform 

 tubercle towards the margin of the central cavity, but at some 

 distance from the edge of the disk. 



This genus appears to differ totally in structure from all other 

 Starfishes. Its greatly flattened disk and rays are built up of an 

 immense number of battledore-shaped calcareous plates, which 

 are most interesting objects for the microscope, on account of the 

 elegance of their form and the beauty of their structure. They 

 are everywhere perforated, except where strengthened by delicate 

 rib-like processes which pass from the shaft to the distal extre- 

 mity of the plate. These plates are laid one over another, both 

 on the oral and aboral surface, like the roofing of a house thatched 

 with palm -leaves ; and the shafts of the plates are buried be- 

 tween the plates of the opposite surface of the disk. Thus the 

 whole structure is built up ; and it is the peculiarly fragile cha- 

 racter of these calcareous plates which makes this Starfish so very 

 brittle when preserved. The fascicles of spines of the sur- 

 face of Palmipes are attached to the calcareous plates, each plate 

 bearing a transverse row of spines across its rounded distal extre- 

 mity. I am not aware that the unique character of the skeleton 

 of this Starfish has previously been noticed. 



Palmipes placenta (Pennant). 



1777. Asterias placenta, Pennant, Brit. Zool. vol. iv. p. 62, no. 50, pi. xxxi. 



fig. 59 A. 

 1783. Asterias membranacea, Retzius, Kong. Vet. Akad. Nya Handl. vol. iv. 



p. 238. 

 1839. Palmipes membranaceus, Forbes, Mem. Wern. Soc. vol. viii. p. 119, 



pi. iii. fig. 3. 



Palmipes placenta is a southern species, found in the Mediter- 



