24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I45 



extend to the peristome in C. prostratus. Apparently this species is 

 smaller than C. subdepressus, although it is possible that no fully 

 grown adults have been collected. C. prostratus resembles in its mar- 

 ginal outline and thick margin Clypeaster ravenelii (A. Agassiz). 

 However, in C. ravenelii the petals are widely open. 



Aberrant specimen. — One specimen is a perfect hexamerous vari- 

 ant belonging to Jackson's (1929, p. 541) group 18. There are six 

 genital pores, six petals (pi. 6, fig. 1), six ambulacral grooves (pi. 



Fig. 17. — Clypeaster prostratus (Ravenel) : Adapical and adoral views of 

 hexamerous variant, U.S.N. M. 648174, from the Recent, Gulf of Mexico, lat. 

 29° 10'N., long. 35° 31' W., Albatross station 2375, X 1. 



6, fig. 2), and six pyramids and teeth (pi. 5, fig. 3). The plate ar- 

 rangement is completely normal (text fig. 17) except that there are 

 two extra ambulacral and two extra interambulacral series. The shape 

 of the test is not regular and the test is not bilaterally or radially 

 symmetrical. The anterior petal (III) can be identified because it is 

 more open than the others. Because of the location of the periproct 

 the petals between it and petal III on the left side of the test are 

 normal. The extra petal is one of those lying between petals V and 

 III on the right side of the test (as viewed adapically). There seems 

 to have been no disruption in the production of plates, for the petals 

 have the same number of pore pairs found in a normal specimen of 

 this size. This aberrant form was evidently not produced by any 

 pathological accident since all the test is hexamerous. It is probably 

 the result of a mutation. 



