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Genus— PYGASTER, Ayassiz. 1834. 



Galerites? (pars), Lamarck. 1816. (?) 

 Clypeus (pars), Phillips. 1829. 

 Echinoclypeus (pars), De Blainville. 1830. 

 Nucleolites (pars), Desmoulins. 1837. 



It is by no means certain that Lamarck was acquainted with the urchins now included 

 in the genus Pyyaster, although he is invariably cited as the author of one of the most 

 typical species of the group, for reasons which will be given in the article on Pyyaster 

 umbrella; it appears that Lamarck's reference to Klein's tab. 12 was an error. In his 

 genus Echinoclypeus, De Blainville associated three distinct types of Echinides, — Clypeus, 

 Pyyaster, and Conoclypeus ; whilst Desmoulins placed the Pyyasters with his Nucleolites, 

 urchins which have limited petaloidal ambulacra, wide poriferous zones, small and irregular- 

 disposed tubercles, with a five-lobed edentulous mouth ; characters which are quite opposite 

 to those possessed by the Pyyasters. The only explanation that can be given for these 

 errors of arrangement by authors of such eminence, is the fact that neither De Blainville nor 

 Desmoulins had seen a Pyyaster, as appears from a note by the latter author, appended to 

 his description of the species in his ' Synonymie generale.'* 



M. Agassiz has the merit of having first detected and pointed out the leading characters 

 of the genus Pyyaster, which he established from a figure of the only species then known, 

 and which has subsequently been proved to be one of the most natural and best-defined of 

 all the genera of the Echinoidea exocyclica. 



The test is sub-pentagonal, more or less elevated and convex on the upper surface, and 

 concave at the base ; the ambulacral areas are narrow, with four or six rows of small 

 tubercles, the marginal rows only extending from the base to the apex of the areas. 



The poriferous zones are narrow, simple, and complete; and the pores are strictly 

 unigeminal throughout. 



The inter- ambulacral areas are in general four times the width of the ambulacral; each 

 of the large pentagonal plates supports numerous tubercles, and those at the border have 

 from six to eight. The tubercles are more or less regularly arranged in vertical and 

 horizontal rows ; it is only the representatives of the two primary rows of each area which 

 extend from the mouth to the disc, all the others disappear in succession on the sides, and 

 the length of each is in proportion to its proximity to the two primary rows. The tubercles 

 are small, and nearly equal-sized ; they are perforated, and raised on bosses with smooth, 

 uncrenulated summits ; depressed, ring-like areolas surround their base ; and the inter- 



* ' Etudes sur les Echinides,' p. 354, No. 2. 



