SYSTEMATIC DESCRIPTIONS CLYPEASTRINA. 



49 



matrix in the two smaller specimens, but in the large one it is preserved in 

 part with traces of the Aristotle's lantern in what appears to be about the 

 middle of the test. 



The material of this species consists of 2 small specimens and a large 

 one. The largest is four-rayed and imperfectly preserved, so that 

 although it is of considerable interest as an abnormal departure from 

 the typical pentamerous symmetry, I hesitate to make it a holotype. 

 The holotype selected is the more perfect of the two smaller specimens. 

 It measures 25 mm. in length, 28 mm. in width, and about 3.5 mm. in 

 thickness. The other small specimen is nearly of the same size. The 

 large specimen, which is four- 

 rayed, is seen from the ventral 

 side with considerable of the test 

 broken away, so that it is a com- 

 bination view of the exterior of 

 the ventral side and the interior 

 of the dorsal side. No complete 

 measurements can be given, but 

 from the periphery to the center 

 it measures 47 mm., so that the 

 whole diameter would be about 

 twice that amount or in the vicin- 

 ity of 94 mm. Three ambulacral 



petals Of the dorsal side are Seen FlG - ^Echinarachnius sebastiani, new spe- 

 , , ... , , ~ cies. Holotype, X 2. 



viewed from within and the 3 are 



placed at right angles to each other in the plane of their main axes ; 

 that is, they present an angle of about 90 to one another instead 

 of an angle of about 72, which is characteristic of the 5 ambulacra 

 of Echinarachnius and other Echini that are not bilaterally elongate. 

 This angle of 90 in the specimen in hand indicates clearly that 

 a fourth petal would complete the circuit of the test. I described 

 (Phylogeny of the Echini, p. 49) a partially six-rayed Recent Echina- 

 rachnius parma, but this is the first four-rayed specimen of the genus 

 known. Six-rayed Echini are exceedingly rare, and I have only seen 

 6 more or less completely six-rayed specimens in a careful examination 

 of some 55,000 specimens of Recent and fossil Echini. More or less 

 completely four-rayed specimens are, however, much commoner. It 

 may be noted here that a four-rayed Clypeaster concavus is figured 

 in plate 2, figure 12, and recently I have seen in the Museum of 

 Comparative Zoology a partially four-rayed Clypeaster rosaceus from 

 the Tortugas, Florida. 



Echinarachnius sebastiani finds its nearest allied form apparently in 

 the Recent Echinarachnius parma (Lamarck). It differs from that 

 species in that it is wider than long and in the extreme thinness of the 

 test, especially on its marginal border. 



